US3111436A - Method of hardening plain carbon and low alloy steels - Google Patents

Description

Nov. 19, 1963 R. F- MCGAVIN METHOD OF HARDENING PLAIN CARBON AND LOW ALLOY STEELS Filed Jan. 15, 1962 United States Patent O 3,31L36 METHQB F HARBENIIG PLAlN CARBN AND LOW ALLQY STEELS Roy F. Mcfavin, Wytheushawe, Manchester, England, assigner to Executors of .l ames Mills Limited, Woodley, near Steelrpol England, a company of Great Britain Filed dan. T15, 62, Ser. No. 166,237 Claims priority, application Great Britain San. 17, 1961 2 Claims. (Cl. ldd- 143) The present invention is concerned with the through quench hardening of a wide range of steel qualities by a special new and improved method. ln particular it comprises the through quench hardening of all plain carbon and low alloy steels of the kind herein referred to as steels of the kind speciiied having carbon contents in the range 0.20% to 0.70% and total contents of alloying elements, including manganese and sincon, in the range up to 9.0% i.e. steels which are capable of transforming almost completely from a full austenite to martensite within the approximate overall temperature range of 359 C. to 170 C.

The quenching media and methods of their application commonly used for hardening steel articles heated to the essentially austenitlc condition have their own characteristic heat removing, or quench cooling powers. These Vary widely from case to case and the media include water, a large variety of quenching oils, heated molten salts or salt mixtures, having low melting temperatures, heated molten metals or alloys with low melting temperatures and a number of aqueous solutions or emulsions of organic or inorganic substances. Among these media, water is one of the most convenient and readily available. It is certainly one of the most powerful quenchants lmown, eing very much more powerful than any commercially used quenching oil, molten salt, salt mixture, metal or alloy. lt is also faster than many of the aqueous solutions or emulsions which have been used as qnenchants. Water is, therefore, one of the very est quenchants available for the hardening of any steel whose alloy leanness or low hardenability, makes high minimum cooling rates through the whole or any part of the overall austenite transformation range necessary for the production o the desired essentially martensitic structural condition in the hardened article. For any given steel cooled from an essentially austenitizing temperature in a quenchant having a certain fundamental cooling power, however great, there will be a limiting sectional size of article above which its ats-quenched structural condition will not be fully martensitic out will begin to show pro-martensitic breakdown products of austenite. IThe incidence of non-martensitic constituents in the quenched structure will increase at the expense of the martensite and the article will hence become less effectively hardened, with continued increase of size or mass. This progressive deterioration in as-quenched structure, which can involve increasing amounts of ferrite and pearlite as well as bainite, deepnding on the type of steel and the range of increasing sizes considered, results in a concurrent deterioration in mechanical properties, especially after subsequent tempering and commences at the most slowly cooling central regions of the particular article, becomes more pronounced there and spreads gradually towards the faster cooling surfaces of the article 3,lll,43d Patented Nov. 19, 1963 as the size is continually increased. As the hardenability of a steel cannot be increased except slightly by enlarging its austenite grain size, itself a dubious practice because of the risk of embrittlement, the only action that can be taken to improve the martensitic hardening of articles in the hyper-critical sizes of the steel concerned is to use a quenchant with a higher cooling power, for example, water.

The conventional largely unrestricted use of water in the hardening of articles made from steels of higher hardenability, or any steel articles at all whose dimensions or sectional masses are small or complicated, is, however, very greatly limited by the risks of cracks occurring in the quenched articles, either during or immediately following the quench, or later, if the steel is left in the hardened and untempered condition for any length of time after quenching. In the case of small or slender articles there is also a grave risk of considerable distortion of shape accompanying unrestricted quenches in water or other aqueous media of similar fundamental cooling power.

The primary object of the present invention is to provide a new or improved method of hardening plain carbon and low alloy martensite forming steels by which certain improvements in the mechanical properties, particularly tensile and impact resistance of a specific steel, may be obtained as compared with an otherwise indenticai steel heat treated by hitherto known methods.

Other objects of the invention will be apparent from the description herein of the advantages believed to be obtained by the application of the invention.

The present invention provides a method of hardening plain carbon and low alloy martensite-forming steels of the kind specified for the purpose of improving their tensile and impact properties, said method comprising heating the steel to an austenitisiug temperature so as to produce an austenitic condition in the steel, effecting a slow cooling of the steel to a temperature lower than said austenitising temperature but high enough to ensure maintenance of said austenitic condition and absence of ferrite or other pro-martensitic precipitations, quenching the so cooled steel with water for a restricted period of time such that the surface layers or case of the steel are transformed substantially completely into martensite with n the core of the steel at such a higher temperature than that of the case as to be still in partly austenitic condition at the end of said quenching operation, and thereupon interrupting the quenching operation and allowing the steel to cool at a slow rate so as to even-up the temperature difference between the case and core and thus ellect light tempering of the case at a case temperature not below C. with continued transformation of the remaining austenite in the core into martensite to an extent such as to produce a mainly lightly tempered martensitic structure throughout the section.

The light tempering of the mainly martensitic core is ensured by the fact that it is an essential requirement of the foregoing method that the evening-up shall take place at an elevated temperature, namely, not below 150 C.

In the present specification certain expressions as used herein are dened as follows:

Low alloy steel-Steels containing alloying elements in addition to manganese and silicon in which the total 3 alloy content including that of manganese and silicon does not exceed 9.0%.

Slow cooling and Cooling at a Slow rata- By these two expressions is meant cooling at a rate rapid enough to maintain an `austenitic structure but at a rate which is less than that obtained by quenching from the austenitising temperature in a liquid `salt bath which is at a temperature of 500 C.

Light tempering and lightly tempered.--Tempering similar to that obtained when steel of the same identical composition after full water quenching, is re-heated to a temperature within the range `of 150 C. to 270 C.

Quenclzing cracks and quench cracking-Any cracks produced in steel articles during or immediately after a quench cooling operation `as a direct consequence of the physical conditions created within the article by the quench.

Delayed or belated cracking-Any cracks occurring in a quenched steel article at some measurable time after quenching has been completed.

Azrstenz'tc condition-An expression commonly used by metallurgists to denote that the steel is substantially completely austenitic; for example the steel may contain 85% austenite.

During the quenching of the vsteel by this invention for a predetermined time period, the surface layers or case of the steel are cooled very rapidly and asymptotically towards 100 C. (the boiling point of water) and are thereby transformed substantially completely into martensite, or at least to a lmixed structure mainly of martensite but with some bainite. The central or core regions of the steel, while also cooled down considerably by this timed water quench are at `'such a higher temperature than the surface, by virtue of the physical properties of the steel and the nature and power of the quench, as to be still in a partly austenitic condition immediately at the Y end of the quenching operation. In the regions between the surface and centre of the steel the balance in the amount of austenite to martensite at the termination of the quench varies with precise position. The rlinal step in the process directly follows the termination or interruption of the quench and is that in which the steel continues to lose heat through its surface and cool down, but at a very slow rate, thus permitting the very substantial evening up, or balancing, lof the immediately post quenching temperature `difference between the case and core of the steel. The heat flowing outwards during this temperature evening up process effects light tempering of the hardened, essentially martensitic surface and sub-surface regions -of the steel and simultaneously procures the continued transformation of the remaining austenite in the core and -intermediate regions largely into martensite which itself thereupon Ibecomes lightly tempered at the evening up temperature.

In any given instance the general level of the evening up temperature is directly dependent on the temperature level at which the initial slow cooling stage was stopped and the water quench commenced and on the exact duration of the timed Water quench.

The present invention provides a new and improved method of hardening plain carbon and low allow martensite forming steels using essentially cool or cold water as the quenching medium and possessing the following advantages:

(1) The achievement on a commercial scale of `generally superior heat treated structures and mechanical properties in steel articles whose dimensions or sectional masses are too large for their elfective hardening by conventionally practiced techniques involving less virulent quenching media. Additionally, the present invention extends the range of article sizes in which certain minimum mechanical property levels can be obtained by quench hardening, or quench hardening followed by tempering.

(2) The substantial reduction in the liability of articles made from hardenable steels of the type described earlier '4 to crack or distort during or after water quench hardening. Additionally, the new quenching technique using water shows a lesser tendency to produce delayed or belated post-quenching cracks in articles hardened in accordance with it and otherwise left untempered than does the application of conventional oil or water hardening, after which conventional hardening, immediate full tempering is highly advisable. Thus the present invention permits of full tempering of the articles after quenching being deferred over a substantial period of time, namely a period of several days or weeks or if desired one or several months in accordance with the particular requirements of the manufacturer and the availability of furnace space at the conclusion of the quenching operation. v

(3) The reduction or elimination of many of the difficulties and inconveniencies generally associated with many conventional commercial quench hardening practices. For example, the reduction of initial and running costs of the medium concerned; the elimination or fire hazards and fume and the simplification of cooling systems in the case of quenches currently involving oils; and the elimination of such post quenching surface decontamination processes as are often necessary after quenches in molten salts or oils.

Although the invention is capable of being applied successfully to the through quenching of articles of small section which can be effectively quenched in oil or other non-aqueous baths, so as to produce a martensitic structure throughout, the important application of the invention is to the quenching of articles of such larger section as are incapable or not readily capable of forming a martensitic structure throughout or -a structure consisting of martensite with some bainite when quenched in oil or other non-aqueous medium.

For any given steel having a chemical analysis within the composition range ydescribed in the first paragraph `of this specicatio-n, the present invention secures all the beneficial' features enumerated earlier concerning the achievement of desirable metal structures and mechanical properties, freedom from cracking and also other safeguards and economies provided:

(l) The initial full `austenitizing is in complete accord with best conventional practice.

(2) rThe immediately subsequent slow cooling is with in the yfully austenitic range `and may be continued, prior to the commencement of water quenching, to a very much lower temperature than that used for the initial austentizing, but consistent with the maintenance of the initial austenitic condition and consequently with freed m from auf p-recipitations `of pro-martensi-tic breakdown products of the austenite.

(3)?he water quench is interrupted after such .time interval as to make the subsequent evening up, `or balancing temperature fall within the overall range 150 C. to 270 C., or preferably, within the more restricted range 200 C. to 225 C. `and most desirably within the range 205 C. to 220 C., the optimum temperature range within the overall range being dependent on the size and shape of the article, and the composition of the steel.

The present invention accordingly seeks 'to exploit the quench cooling power of water, by applying it to the numerous through hardening steels envisaged over just 'those ranges of temperature in which its speed of cooling is most valuable in suppressing las far as possible the proarnartensitic breakdown `of austenites and hence in securing maximum 'hardening efficiency. At the same time, despite the `substitution of the powerful water quenching for oil or other milder quenches, the features of delaying the yonset of these water quenches to selected temperature levels considerably below those used in normal practice and Iof interrupting the quenches after predetermined time intervals in accordance with this invention both greatly reduce the risks of cracking `and distortion occurring during the vigorous quenching. The said features of the process achieve this desirable end by greatly reducing the thermal gradients and stresses and also the transformational stresses which are set up lduring the water quench itself. Additionally, the evening up of temperature, or heat balancing, which takes place naturally immediately after the termination of water quenching, secures 4a light tempering yof the quenched structure of a steel during the subsequent very slow final cooling stage of the process. This light :tempering minimizes the risk of delayed cracking, as distinct from quench cracking, occurring at any time after the quench.

It is an essential feature of the present invention that at no time after the commencement of the interrupted water quench itself is there any deliberate holding of the steel lat selected temperatures. At all times `after starting the quench, heat is being removed from lthe surfaces of the steel and the temperatures are not held constant. This continuons heat removal effectively assists the smooth and continued transformation to martensite or to martensite with some bainite of the significantly austenitic material of the hot core regions during the post-quenching evening up stage of the process and immediately thereafter. Further, in many applications there will be no holding of the steel at the `selected start of quench temperature either, and in such cases the cooling will be continuous throughout the process. In certain instances, however, a short arrest at the start of quench temperature may 'be arranged as a matter of practical convenience, as explained below.

The quenching of the steel with water may be eliected by directing sprays `or jets of water onto the steel or it may be effected by plunging the steel into a bath of water.

In the application of the technique of delayed and interrupted water quenching to general practice, the articles involved will be treated either singly, which are the simplest cases, or by some bulk method involving several articles at one time. ln applications of the latter form it is essential that adequate provision be made to ensure that, within reasonable limits, each article cools and generally behaves during the overall hardening process like all the others constituting the load. While it is not necessary to tr] to adjust the individual items in a load so that each behaves thermally in the way a single completely isolated article would do in similar circumstances, -it is, nevertheless, essential to avoid closely packed or closely interlocking arrangements of the individual items which would give rise to significant thermal differences between them. It is also essential to ensure that all of the articles in a load complete the preliminary stage `of the process, i.e. slow cooling from the initial austenitizing temperature to the appropriate start of quench temperature, before the interrupted water quench `is commenced, for example, by immersing the articles in a salt bath at a temperature corresponding to the selected start of quench temperature, so as necessarily to arrest the cooling for a short time before quenching.

In its application to quench hardening practice We have found that important features of delayed and interrupted water quenching reside in the following limitations of its general principles.

(l) While the preliminary air or other slow cooling from the initial austenitizing temperature can be terminated at any lower start of quench temperature, provided the austenite remains sufficiently stable to persist completely to the chosen level, in the performance of the invention it is found most convenient to commence the quenching operation from one of three gro-ups of temperature ranges namely:

7l5/735 C., 645/660 C. and 595/610" C.

These temperature ranges eectively cater for the very varied austenite stability characteristics of all the Steels encompassed by this invention over a wide range of preliminary slow cooling rates and they allow for the small temperature differences which exist within and between articles after such cooling. At the same time they permit the steels to be water quenched from very much reduced temperature levels which ensure a high degree of safety from quench cracking and distortion.

The 715/735 C. level can be used as a start of quench temperature for all of the steels of the kind specified, but is the minimum temperature level applicable to the through hardening steels of lowest hardenability and least austenite stability during preliminary air cooling, viz. the plain carbon and carbon-manganese steels.

The 645/660 C. and 595/61G C. levels as start of quench temperatures are restricted to alloy steels having respectively intermediate and higher hardenability and having respectively intermediate and high austenite stability during preliminary air or other slow cooling from the initial austenitzing temperature.

C re and experience must be exercised when deciding which of these three levels should be used in the application of the invention to any particular steel. As a general principle, it is always highly ads/isable to slow cool from initial austenitizing to the lowest of the three start of quench temperature levels permitted by the austenite stability characteristics of the particular steel at the slow preliminary cooling rate involved.

If the 715/735o C. temperature range is used as a start of the ,quenching temperature, there is no minimum limit lwhich is of practical interest to the rate of slow cooling of the steel from the austenitizing temperature but if either of the above specified lower start of quenching temperature ranges are employed then with some of the steels to which the lower temperature ranges are applicable, it is necessary to effect the slow cooling from the austenitizing temperature at a rate not less than that of ordinary conventional air cooling to avoid 'any breakdov/n of the austenite to promartensitic products.

(2) In heat treatment practice the present invention will be particularly applicable to the through hardening of solid steel bars and other long solid articles of simple, regular, symmetrical rand convex section, however produced. When such articles have lengths which are great compared with their cross sectional dimensions, ie. 'when their length to diameter ratios exceed about 8 to l, the heat losses from their end faces become negligible compared with those from their lengthwise or circumferential faces, at all stages of .the delayed and interrupted water quenching process. If, `in addition, the preliminary -slo-w cooling of such bars from an initial austenitizing temperature to a Selected start of quench temperature is la normal air cool, then the following empirical law has been found to apply with sufficient accuracy and scope for practical purposes.

Time of air cool T AC (minutes) :KD Where D=bar diameter for rounds.

Time of air cool TAC (minutes)=K (A/F) where A/ F :across `lia-ts measurement for hexagons, octagons, squares etc.

where K is a constant, the value of which depends on the particular steel concerned.

(3) In like manner, the interrupted water quenching of steel bars from any selected start of quench temperature to any desired end of quench evening up temperature level follows the empirical law given below with very considerable accuracy and reliability.

Time of interrupted water quench TIWQ (secs.)r=kl)2 -Where D=bar diameter for rounds.

Time of interrupted water quench TIWQ (secs.)=k (A/F)2 :where (A/F)-=across ilats measurement for hexagons, octagons, squares etc.

where k is a constant, the value of which depends on the particular steel concerned.

The simple equations shown in items 2 and 3 above hold the great merit of making temperature measurements during the overall cool-ing cycle quite unnecessary for guiding the counse of the practical applications of the invention. By their use it is only necessary in any given case to know the initial austenitizing temperature and the values of the proportionality constants in the two equations.

The exact numerical values to be ascribed to these constants depend on such local plant conditions as the temperature levels selected `for initial austenitizing, for starting the interrupted water quench and for iinal elvening up immediately after the quench and also the broad nature of the external surfaces of the steels, i.e. whether they are smooth or rough, bright and clean or black, dirty or scaly. The chemistry of the steel within the limits envisaged in this specification will have only a minor influence on the values of the constants and can be ignored. For any given unit and set oi typical working conditions, practical values can rapidly be obtained for the constants lfrom a few simple, pilot experiments involving careful temperature and time measurements. Having thus been established, the constants can be used for all cases thence forward until a major change occurs in the conditions of operation.

The short table set out below gives the details of the application of the present invention to the quench hardening of short i.e. 18 .inch long, accurately sized, bright iinished solid steel bars of simple, regular symmetrical and convex cross section which have been heated for full austenitizing at `850" C. in a conventional neutral salt bath plant. -ln the table are set out the values of K and k for the examples quoted.

Austentzng Temperature in Neutral Salt Bath 850 C.

STEEL QUALITY GROUPING [Austcnite Stability Rating During Preliminary Slow Cool] Low Carbon High Low and Carbon Intermediate Alloy Steels Manganese Low Alloy EN 16, 17, 23, Steels EN 12, Steels EN 24, 25, 26, 100, 18, and 111 19 110, and similar steels Air Cooling Time in mins. 1.10D (KD) 1.90D (KDL 2.6D (KD).

to Start of (in terms of inches diameter). Start of Quench Tempera- 715 to 735 C 645 to 660 C. 595150 610 C.

ture used, Interrupted Water Quench 925D2 (kD2) 8.75D2 (lcD2). 825D2 (kD2).

time in secs. (in terms of inches diameter). Evening up Temperaturc 205 to 220 C 205 to 220 C 205 to 220 C.

The EN reference numbers given as examples of steel qualities in this table and elsewhere in this specification are taken from lthe British Standards Specication 97021955 published by The British Standards Institution.

The main feature of the quench hardening process, which is the subject of the present invention, can be schematically represented in a generalized form for an lapplication in which the temperature distributions are simple and regular. The accompanying diagram illustrates such a case, namely that of a long, solid steel bar of circular cross section and having a chemical composition within the ranges set out in the opening paragraph of this specicatio-n.

The diagram depicts a transverse section through the bar in such a way that the vertical side lines represent the cylindrical surface of the bar, while the central vertical lines represent the central longitudinal axis of the bar. All lateral or horizontal lines on the diagram, whether dotted or solid, straight or curved, represent temperature levels or distribution. More specilically I show:

Solid isothermal Lines 1-1 (780 C. to 930 C.) are the appropriate limits of the normal austenitizing range for dl the steels involved, prior to quench hardening. 810 C. to 860 C. is the most widely used portion of this overall range.

2-2 (350 C. to 280 C.) lare the approximate limits peratures, i.e. the temperature at which martensite starts Y forming.

3 3 (245 C. to 170 C.) are the approximate limits of the normal range for all the steels involved of M9074 temperatures, i.e. the temperature at which the steel is of a mainly martensitic structure, ie, at least martensite.

4. C. is the temperature of boiling water, to which the falling surface temperatures of bars tend to become asymptotic during interrupted quenohes.

Dotted Isothermol Lines and Dotted Curves 735 C. and 715 C., 660 C. and 645 C. and 610 C. and 595 C. are shown as the limits of the three start of quench temperature brackets selected for general use and described in detail in the foregoing text of this specilication.

As previously :described the approximate range 735 C. to 715 C. can be used for all the steels envisaged herein, but is mainly reserved for the plain carbon and carbonrnanganese types and steels of low overall hardenability like EN l2, 18 and lll. The approximate range 660 C. to 645 C. is used for low alloy steels of intermediate overall hardenability like EN 19 and the approximate range 610 C. to 595 C. is likewise generally reserved for the higher hardenability, low alloy steels of Ithe EN 16, 17, 23, V24, 25, 26, 100 and 110 type.

Entirely within these temperature brackets are shown respectively the three shallow dotted curves, A, B and C. These curves represent the approximate temperature distributions across the sections of bars at the end of the preliminary air cool from the initial austenitizing and immediately prior to starting interrupted water quenching.

The dotted curve D indicates the approximate distribution ot temperature across the sections of round lbars at the instant of ending interrupted water quenching, when the quenching time has been arranged for subsequent evening up, of balancing off, of temperature within Ithe range 205 C. to 220 C. For the case of round bars the curves D are similar to parabolae.

The desired evening up temperature bracket 205 C. to 220 C. is also shown, cross hatched, in the diagram.

The tables Nos. l, ll and llll set out below give the mechanical 'test results obtained on solid bars of various types of through hardening steel after good commercial oil quenching and tempering and after interrupted water quenching in accordance with the general principles of the present invention. rlhe figures demonstrate the beneiits of delayed and interrupted water quenching from the viewpoint of mechanical :tests and also illustrate some of the other points raised in the foregoing script.

NOTES: ln the tables below the following abbreviations are used:

Aust. Austenitizing--the operation prior to any hardening.

SQT. Start of quench temperature.

IWQ to ET interrupted water quenched to evening up temperature.

FOQ Fully oil quenched until cold.

FT Final tempering temperature prior to mechanical testing.

CON Test piece taken from centre of bar.

ECC Test piece taken "from near bar surface.

The tensile and lzod impact test specimens were machined from the central axis of the trial Vbars unless otherwise stated and where the size of the material has permitted the testing of both the central axis region and an area nearer the lbar surface, in fact lroughly the mid radial region, the figures are headed respectively by the abo-ve abbreviations Con and Eco In all the cases quoted the vbars were normally air cooled from the initial austenitizing temperature to the start of a quench temperature range selected.

TABLE I Carbon Manganese Steels DETAILS OF ANALYSIS McQuaidEhn Steel Code C, Percent Mn, Percent S, Percent P, Percent Si, Percent Ni, Percent Cr, Percent Mo, Percent Austenite gram sxze 1 0. 47 1. 39 0. 034 0. 048 0. 137 0. 04 0. 02 0. 01 2 t0 4 2 0. 45 1. 55 0.031 0. 044 0.188 0.07 Nil 0.02 2 t0 3 MECHANICAL TEST RESULTS Ratio Elonga- Reduc- Yield Mex. Yield: tion pertion of Steel Code and Original Form Heat Treatment Stressl Stress 1 Max. cent on Area, Izod Stress, 4 #Tea Percent Percent Algst. S40/850153., SQ1()715/7:`5 C., IVQ t0 47.6 59.0 80.8 19. 5 53. 6 42, 41, 39 T218 C. T 600 1 1/ 4" dm' bar Hot Forged- Aust. S40/850 c., sQT 715/735o C., FOQ, as. 0 57. s 65. 7 22.0 40. o 15,11,10

FT 560 C.

Alllst. S35/8(4)() CT, SQTC715/735 C., IWQ t0 50.0 59.6 84.2 21.0 56.0 5l, 49, 46 *T225 F 585 11'1/2" ma' bar H0 FOfgcd Aust. S35/840 C., sQT 715/755 C., FOQ, 30. s 54. 4 67. 5 25. 0 54. s 19,111,15

FT 585 C.

Aust. 840,8(530 CT, SQoTC715/735 C., NVQ, to 46. 6 58.0 80.2 21.0 49. 6 47, 42, 42 u ET211 ,F 735 2 2" dlabm" H01: Embed Aust. S10/850 C., sQT 715/755u C., FOQ, 41. 0 01. 2 0s. 0 17. 0 35. 4 1511,13

FT 600 C.

1 Tons per square inch.

TABLE 1I Alloy Steels DETAILS OF ANALYSIS Steel Code C, Percent Mn, Percent S, Percent P, Percent Si, Percent Ni, Percent Cr, Percent Mo, Percent 1-EN 19 0. 40 0. 69 0.045 O. 022 0. 188 0. 14 1. 23 0. 36 2-EN 19 0.34 0.72 0. 026 0.017 0. 24 0.23 1.36 0.37 3-EN 16- 0.35 1. 55 0. 034 0.025 0.244 0. 25 0.14 0.36 4-EN 19 0.36 0. 68 0. O17 0.013 0.31 0.23 1. 27 0.39 -EN 12 0. 39 0. 82 0. 048 0. 041 0. 16 0. 74 0. 21 0. 04

MECHANICAL TEST RESULTS Ratio 0.1 0.1 Per- Percent Elonga- Reduc- Steel Code and Original Form Heat Treatment cent Proof Max. Proof: tien Pertion oi Izod Stress 1 Stress 1 slttax. cent on area,

ress Percent Percerft 4 w/Area Aust. 850 C. Ecc 57. 0 65. 3 87. 2 20. 5 58. 2 59, 63, 64 SQT 595/610 C. C011., IYVQ to ET 223 C., 52.0 62.6 83.0 24.0 58.0 68, 66, 67 12.58 dia. ber, Black Rolled FT 650 C.

Aust. 850 C. Ecc 53. 5 67. 3 79.0 21.0 59.0 59, 56, 65 SQT 850 C. Con. FOQ, FT 620 C 47. 5 61. 9 76. 8 19. 0 53. 5 33, 33, 28 Aust. 850 C. Ecc 78. 5 86. 0 91. 3 14. 5 46. 8 31, 32,33 SQT 595/610 C. Con., IWQ to ET 232 C., 72.0 84.0 85.8 14.5 41.3 33, 31, 35 `2.3/8 da. bar, Bright FT 520 C.

Aust. 850 C. Ecc 79.0 93, 6 84. 6 42.0 18,16, 19 SQT 850 C. Con. FOQ, FT 510 C 69.0 85. 0 81.2 42.8 20, 23, 22 Aust. S50/855 C., Ecc 59. 3 66.0 90. 0 10. 0 17. 3 60, 61. 59 SQT 595/610 C. Con., IWQ to ET 215 C., 49. 5 60.0 82. 7 16.5 37.7 57, 56,67 3-3 square, Black Billet FT 610/620 C.

Aust. S50/855 C., Ecc 52. 5 62. 5 84. 0 16. 5 40. 4 46, 45, 56 SQT 850 C. Con. FOQ, FT 580 46.0 61.6 73. 8 16. 5 36. 8 39, 45, 45 Aust. 855 C. Ecc 60. 4 70. 3 85.8 18.0 54. 7 55, 53, 54 SQT 595/610 C. C011., NVQ t0 ET 230 C., 50.3 63.1 79.7 17.0 46.4 50,54, 60 4-4 square, Black Billet FT 630 C.

Aust. 850 C. Ecc 56. 3 71. 6 78.8 19. 0 55.1 32, 37, 50 SQT 850 C. Con. FOQ, FT 600 C 49. 5 65. 2 75. 8 15.0 40.0 21, 24, 20 (`0.,g$9 715/735 C., IWQ to 41. 5 55.2 75.0 22. 0 51. 6 73, 57, 63 0 5 0 52"d1'baffnght Aust:j 50 c. con., sQT 850 o. FoQ, FT 57.2 54.0 69.0 43.3 59, 50, 50

l Tons per square inch.

1 l 172 TABLE Ill Direct Production Trials on Bars of Length Varying Between 12 and 17 Feet ANALYSIS OF STEEL INVOLVED C, Mn, S, P, Ni, Cr, Mo, Percent Percent Percent Percent Percent Percent Percent Percent EN 161W, 2.15/16" da.. and 1.950 dia., Black Rolled Bars i. 38 1. 56 180 042 .074 21 10 .23 EN 16, 2.580 dia. and 2.5/16 die., Black Rolled Bars .37 1. 64 .033 .026 22 .16 .10 .27

l/IECHANICAL TEST RESULTS Ratio Yield Max. Yield: Elongation Steel Type and Form Heat Treatment Stress l Stress 1 Max. Percent on Izod SIQSS, 4 .`/Area Percent Aust. S50/860 C., C011- 50.0 59. 4 84. 2 19. 5 42, 44, 44 SQT (S45/660 C., Con., IVQ 49 Secs., FT 52.1 60.4 86.2 20.0 45, 43, 42 EN 16M 2.5/16" da., Black Bar AS 580 C.

Rolled. Alb" C., Con., SQT 830 C., FOQ, FT 40 48. 8 81.9 23.0 41, 50, 49 ArigbSCB" C., Con., SQT 830 C., FOQ, FT 44 52 84.7 20 26, 23, 25 EN 16M, 1.950 dia., Black Bar As Aust. S50/860 C. C011 50 5S. 4 85.6 20 50, 45, 46 Rolled. SC()Io65/660 C. Gon., IQ 35 Sces., FT 48 57. 2 84.0 23 49, 52, 48

Aust. S50/860 C. Ecc 63. 8 21.0 55, 56, 56 EN 16,(12580 die., Black Bar AS SloT/H" C. Ecc., IW Q 65 Sees., FT 56 64.0 87. 5 19,0 54, 54, 55

Rolle Alt.o 8550" C. Con., SQT 850 C. FOQ, FT 50 59.6 84.0 21.0 33, 28, 32

EN 16, 2.5/16 da., Black Bar As {Ause 850 C. Con., SQT 715/735 C., IVVQ 52 52.0 59.4 87.2 22. 65,69, 68

Rolled. Secs., FT 600 C.

l Tons per square inch.

As will be seen, the foregoing tables set out a compari# son between the physical properties of a number of different steels which in each case are subjected to the two contrasting treatments of delayed and interrupted Water quenching by the :method according to the present invention and oil quenching as hitherto customary.

Attention is directed to the marked improvement in the impact resistance as denoted by the Izod figures of steel heat treated in accordance with the present invention as compared with the conventional heat treatment.

Such improvement in Izod figures is in general accompauied Aby an improvement in the percentage reduction in area in the tensile test.

Further the tensile test figures in practically every case show a marked improvement in yield point and in the ratio of yield to maximum stress for the materials treated according to the present invention, as compared with those hardened by the conventional direct oil quenching methods.

The micro-examination of steels hardened in accordance with the present invention disclosed structures pre dominantly of tempered martensite in the fully treated condition. In some cases a little tempered Bainite was observed in the central or core regions of the bars. This was found later to have been caused partly by .the evening up temperatures having been generally a little higher than desired and also, due to the comparative crudity of the actual experiments, somewhat irregular. Despite this over 90% martensite was vgenerally observed in the various examinations.

'ln some cases, particularly in the EN 19 steels, a little free fermite was noted in the micro-structures, but generally speaking the amount was not at all substantial. The presence of this ferrite was later found to have been caused by pre-coolingthe :steel from the initial austenitizing temperature to a start of quench temperature which was unduly low for the steel concerned and which had therefore permitted a little ferrite separation from the parent austenite to occur prior to quenching. It was precisely on account of this breakdown, that the start of quench temperature ran-ge 645/ 660 (L -not used in the test trials Whose results are recorded here-is the preferred start of quench temperature range for alloy steels, having intermediate hardenability and intermediate austenitic stability during preliminary slow cooling.

With the exercise of the generally closer experimental controls just referred to, still more superior mechanical properties and micro structures would have been obtained in some of the cases quoted.

ln addition to the advantages already specified the present invention enables mechanical properties to be developed in plain carbon and low alloy steels, which are comparable with the mechanical properties developed with existing heat treatment techniques by otherwise similar steels Aof a higher alloy content.

Thus the present invention enables a steel of a lower alloy content to be substituted for one of ya higher alloy content, without any reduction in the desired mechanical properties of the pant in question; and in the case of steels of a very low alloy content at the lower end of the low ailoy ran-ge, the present invention enables a plain carbon steel to be substituted; without any reduction in mechanical properties.

Having regard to the high cost yof alloy steels, the saving in cost of the steel by reduction lin quantity of the alloying eiement used, yfor a given performance of the steei, is considered to be of `great practical signicance.

The actual composition `of each lof the EN steels herein specified as set out in the British Standards Speciiication 970: 1955 published by the British Standards institution is for convenience tabulated below `as follows:

CHEMICAL COMPOSITION-EN 12 CHEMICAL COMPOSITION-EN 16 CHEMICAL COMPOSITION-EN `100 Percent Percent Element Element Min. Max. Min. Max.

0. 050 0. 050 CHEMICAL COMPOSITION-EN 16M 0. 0. 40 0.25 1. 30 1. 80 0.115 0.20 0. 0. 35 0.12 0.20 0. a0 0.050 1. e0 1. 0,20 CHEMICAL COMPOSITION-EN 17 0. 050 0.050

0. 10 0. 35 1. 80 0.55 0. 050 0. 40 0. 050 0. 35 0.90 1.50 0.75 0, 050 0. 050 0. 0.35 0.95 30 NOTE: In each of the foregoing EN compositions iron (Fe). and the 1.15 usual commercial impurities constitute the remaining constituents of 0. 050 the steel. 0'050 What lI claim then is:

1. A method of hardening plain carbon and low alloy martensite-forn1ling steels for the purpose of improving 0 A5 35 their tensile and impact properties, said method coin- OV prising the steps of (1J-gg (a) heating the steel to an auste-nitising temperature 0:40 so as to produce an aiistenitic condition in Ithe steel, ggg 40 (b) subjecting the steel to cooling to a temperature be- -low the austenitising temperature and high enough n I .y CHEMICAL COMPOSITION EN 23 for the steel to lml still 1n austenitic condition and free from ferrlte and other pro-marterisitic precipita- 0 25 0 35 tions, the cooling irate being rapid enough to main- 0110 i135 tain an austeni-tic structure but less than that obg ggg 45 tained by ouenching from the austenitising tempera- 0, 1 00 ture in a liquid salt bath which is at a temperature ggg() of 500 C., P 01050 (c) quenching the so coo-led steel in Water so as to transform the sur-face layers of the steel substan- CHEMICAL COMPOSITION-EN 24 50 nelly completely into martensite,

(d) interrupting the water quenching after a time in- 0. 35 0.45 terval such that the core of the steel is at a temperaggg g' ture higher than the surface layers and is trans- 1. 30 1.80 Iformed only partially into martensite so `as substangjgg 'g 55 tially to comprise austenite and martensite, 0.050 -(e) taereupon continuing the cooling of the steel at O'OEO a rate slower than that of said water quenching so CHEMICAL COWPOHIWIO T v as to even up the temperature dierence between l l' D l NH 'a the surface layers and the core, and

0 Q r 60 (f) thus etect light tempering of the surface layers Oj g: at a temperature not below 150 C. with continued ggg $.70 .transformation of the remaining austcnz'te in the 0:50 Ojg core into martensite to an extent such as to pro- 0.40 1.79 duce a Imainly lightly tempered martensitic strucjj Ofggg ture throughout the section.

2. A method accoidinfr to claim l wherein the water CHEMICAL COMPOSTION EN 26 quench is interrupted after a time interval such that the temperature of evening up of the surface layers and 0,36 0 44 core is Within the range of 200 to 225 C.

e g' 1g g- 70 3. A method according to lclaim l, wherein ,the ternj 2:50 gjo perature of evening up of the surface layers and core is gig ggg Within the range of 205 to 220 C1 0,'050 4. A method according to claim 1 wherein the steel is 0-050 in the form of an elongated solid article having a length at least eight times its diameter.

5. A method according to claim 4 and wherein the preliminary slow cooling .is a normal air cool, characterised lin that the time of air cooling is directly proportionate to the diameter of the article.

6. A method according to claim 4 and wherein the article is a round bar, characterised in that the time of the interrupted water quench is `directly proportionate to the square of the bar diameter.

7. A method according to claim 4 and wherein the article is in the lform of a oer having an even number of at faces not exceeding eight in number spaced equiangularly around rthe periphery of the bar characterised in that the time of the interrupted water quench `is directly proportion-ate to the square of the thickness of the bar as measured between two or-posite hat face 8. A method according to c1 un 1 wherein the steel is heated to an austenitising temperature within the range of 810 C. to 860 C.

9. A method according to claim l wherein ait-er the light .tempering the steel is subjected to yfull tempering after a time int al, several days, which is greater than that required if lfull tempering is elected immediately after ythe light tempering.

l0. A method according to claim 1 wherein after the light tempering the steel is subjected to `ull tempering after a time interval, e.=g. several days, which is greater than that required if full tempering is elected immediately after the light tempering.

ll. A method of hardening plain carbon martensiteforming steel having a carbon content within the range 0.20% to 0.70% and capable of transforming substantidly completely from a -full austenite to martensite within the yoverall temperature range of 350 C. to 170 C., said method comprising the steps of:

(a) heating the steel to an austenitising temperature so as to produce an austenitic condition in the steel,

(b) cooling the steel in air Ifrom said austenitising temperature to a llower temperature within the range 715 C. to 735 C.,

(c) quenching in water the steel vwhen at a temperature within said temperature range of 715 C. to 735 C.

(d) interrupting the water quenching of the steel after a time interval such that the sur-face Ilayers of the steel are substantially completely martenr'fic and the core of the st el is martensite and auste ite, and

(e) allowing the steel to :cool in air `to even up the temperature difference between the surface layers and the core while effecting light tempering of the surface layers at a temperature not below 150 C.

l2. A method of hardening piain carbon and low alloy l-'rnartensite-forming steels for the purpose of improving their tensile and impact properties, said method comprising the steps of (a) heating the ste-el to an austenitising temperature so to produce an austenitic condition in the steel,

`(g) subjecting the steel to cooling from said austenitising temperature to a lower temperature with- -in the range of 715 C. to 735 C., the cooling rate bein-g rapid enough to maintain an austenitic strucvture but less than that obtained by quenching from the austenitising temperature in a liquid salt bath which is at a temperature of 500 C.,

(11) quenching `in water the so cooled steel when with- .in said temperature range so `as to transform the surlface layers of the steel substantially completely into martensite,

(d) interrupting the Water `quenclring 4iter a time interval such that the core of the steel is at such a ternperature, which is higher than the surface layers, that the core is trans-formed only partially into martensite so as substantially to comprise austenite .and martensite,

16 thereupon conti the cooling of the steel at rate siower than that yof said Water quenching so as to even-up the temperature dille-renee between the surface layers and the core and (i) thus elect light tempering of the surface layers at a temperature not below C.

13. A :method of hardening a low alloy rnnrtensiteforming steel or intermediate hardenability and intermediate austenite stability when air cooled from austenitising temperature, which steel has a carbon content in the range of 0.20% to 0.70% and total coutent of ailoying elements including manganese and silicon, not exceeding 9.0%, said method comprisi `g the steps of:

(a) heating the steel to an austenitising temperature so as to pro-duce an austenitic condition in the steel,

(j) cooling steel in air from said austenitising temperature to a Vlower temperature within Athe range 645 C. to 735 C.,

(it) quenching in water the so cooled steel when within said temperature range so as to transform the surface layers of the steel substantially completely into rnariensite,

(d) Linterrupting the water quenching after la time interval such that the core of the steel is at such a temperature, which is higher than the sur-tace layers, that the core is transformed only partially into martensite so as substanti lily to comprise austenite and martensite.

(e) thereupon continuing the cooling of the steel at a rate slower ,than that of said Water quenching so as to even-up the ltennperature difiere-nce between the surface llayers and the core and (i) thus effect light tempering of the surface layers at a te .rpcrature not below 150 C.

14. A method according to clim'rn 13 twhere the steel is quenched in water when at a temperature Within the ran-ge of 645 C. to 660 C.

15. A method of hardening a low alloy martensiteforming steel of `high hardenability 'and .high austenite stability when air cooled from .-austenitising temperature, which steel has `a carbon content in the range of 0.20% to 0.70% and total content of alloying elements including manganese and silicon, not exceeding 9.0%, said method comprising the steps of:

(a) heating the steel to an austenitising temperature so as to produce an austenitic condition in the steel,

(k) `cooling the steel in Eair from said austenitising temperature to a lower temperature within the range 595 C. to 735 C.,

(h) quenching in water the so cooled steel when within steel temperature range so as to transform the surface layers of the steel substantially completely into martensite,

(d) interrupting the lwater quenching after a time interval such that the core of the steel is at such `a temperature, which is higher than the surface layers, that the core is transformed only partially into martensite so `as substantially to comprise austenite and martensite,

(e) thereupon ycontinuing the cooling of the steel at a rate slower than that of said water quenching so as to even-up the temperature ditercnce between the surf-ace layers `and the core and (i) thus effect light tempering of the surface layers at a temperature not below 150 C.

16. A method according to claim 15 'Where the steel is quenched in Water when at a temperature ywithin the range of 595 C. to 610 C.

17. A method of hardening plain carbon martensiteforming steel having a carbon content within the range 0.20% to 0.70% and capable of transforming substantially completely `from a full yaustenite to martensite Within the overall temperature range of 350 C. to 170 C., said method comprising the steps of (a) heating the steel to an austenitising temperature so as to produce an austenitic condition in the steel,

(b) cooling the steel in ail from said austenitising temperature to a lower temperature within the range 715 C. to 735 C.,

(d) interrupting the Water quenching or the steel after a time interval such that the surface layers of the steel are substantially completely martensitic and the core of the steel is martensite and austenite and (m) lallowing the steel to cool in air until the temperature of the surface layers and core even-up within the temperature range of 200 to 225 C.

18. A method of hardening a low yalloy martensiteforming steel or" intermediate hardenability and intermediate austenite stability when air `cooled from austenitising temperature, which steel has a `carbon content in the range of 0.20% to 0.70% and total content of alloying elements including manganese and silicon, not exceeding 9.0%, said method comprising the steps of:

(a) heating the steel to au austenitising temperature so as to produce an austenitic condition in the steel,

(j) rcooling the steel in air .from said austenitising temperature to a lower temperature within the range 645 C. to 735 C.,

(lz) quenching in Water the so cooled steel when Iwithin steel temperature range so as to transform the surface layers of the steel substantially completely into martensite,

(d) interrupting the water quenching after a time interval such that the core ot the steel is at such a temperature, which is higher than the surface layers, that the core is .transformed only partially into martensite so as substantially to comprise austenite Iand martensite, and

(m) allowing the steel to cool in air until the temperature or" the surface layers `and core even-up within the temperature range of 200 to 225 C.

19. A method of hardening a low alloy martensiteforming ste-el of high hardenability and high yaustenite stability when air cooled from austenitising temperature, which steel has `a carbon content lin the range of 0.20% to 0.70% `and total content of alloying elements including manganese and silicon, not exceeding 9.0%, said method comprising the steps of:

(a) :heating the steel to `an yaustenitising temperature so -as to produce an austenitic condition in the steel,

(k) cooling the steel in :air from said austenitising tem'- perature to a lower temperature Within the range 595 C. to 735 C.,

(h) quenching in water the so cooled steel when Within steel temperature range so as to transform the surface layers of the steel substantially ycompletely `into martensite,

(d) interrupting the water quenching after a time interval such that the core of the steel is at such 'a temperature, which is ihigher than the surface layers, that the core is transformed only partially into martensite so las substantially to comprise 'austenite and Inartensite, and

(m) `allowing the steel to cool in air until the temperature of the surface layers and core even-up within the temperature range of 200 to 225 C.

20. A method of hardening plain carbon and low alloy martensite-forming steel of the kind specified, for the purpose of improving their tensile `and impact properties, said method comprising the steps of (a) heating the steel to an austenitising temperature so as to produce an austentic condition in the steel,

(n) subjecting the steel to cooling to a temperature within the range of 735 C. and 595 C., depending on the composition of the steel, and high enough for the steel to be still in austenitic )condition and free from ferrite and other pro-Inartensitic precipitations, the cooling rate being rapid enough to main- .tm'n an austenitic structure but less than that obtained by quenching from the austenitising temperature in a yliquid salt bath which is at a temperature of 500 C.,

(c) quenching the so cooled steel in water so as to transform the surface layers of the steel substantially completely into martensite,

(d) interrupting the water quenching alter a time interval such that the core of the steel is at a temperature higher than the surface layers and is transformed only partially into maitensite so as substantially to comprise austenite and martensite,

(e) thereupon 'continuing the cooling of the steel at a .rate slower than that of said water quenching so as to even up the temperature difference between the surface layers and the core, and

(f) thus effect light tempering of the surface layers at a temperature not below C. with continued transformation of the remaining austenite in the core into martensite to an extent such as to produce a Vmainly lightly tempered martensitic structure throughout the section.

2l. A method of hardening plain carbon and low alloy martensite-forminig steels for the purpose of improving their tensile and impact properties, said method comprising the steps of (a) heating the steel to an austenitising temperature so as to produce an austenitic condition in the steel,

(n) subjecting the steel to cooling to a temperature within the range of 735 C. to 595 C., depending on the composition of the steel, and high enough for the steel to be still in austenitic condition and free from ferrite and other pro-martenstic precpitations, the cooling rate being rapid enough to maintain an austenitic structure but less than that obtained by quenching from the austenitising temperature in a liquid salt bath which is at a temperature of 500 C.,

(c) quenching the so cooled steel in water so as to transform the surface layers ofthe steel substantially completely into mantensite,

(o) interrupting the water quenching after a time interval such that the surface layers and core of the steel are respectively at a temperature of below 150 C. and below 350 C., and

(p) continuing the cooling of the steel at a rate slower than that of said water quenching to even up the temperature of the surtface layers and core at an evening-up temperature within the range of 150 C. to 270 C.

References Cited in the le of this patent UNITED STATES PATENTS

Claims (1)

1. A METHOD OF HARDENING PLAIN CARBON AND LOW ALLOY MATENSITE-FORMING STEELS FOR THE PURPOSE OF IMPROVING THEIR TENSILE AND IMPACT PROPERTIES, SAID METHOD COMPRISING THE STEPS OF (A) HEATING THE STEEL TO AN AUSTENITISING TEMPERATURE SO AS TO PRODUCE AN AUSTENITIC CONDITION IN THE STEEL, (B) SUBJECTING THE STEEL TO COOLING TO A TEMPERATURE BELOW THE AUSTENITISING TEMPERATURE AND HIGH ENOUGH FOR THE STEEL TO BE STILL IN AUSTENITIC CONDITION AND FREE FROM FERRITE AND OTHER PRO-MARTENSIC PRECIPITATIONS, THE COOLING RAE BEING RAPID ENOUGH TO MAINTAIN AN AUSTENITIC STRUCTURE BUT LESS THAN THAT OBTAINED BY QUENCHING FROM THE AUSTENITISING TEMPERATURE IN A LIQUID SALT BATH WHICH IS AT A TEMPERATURE OF 500*C., (C) QUENCHING THE SO COOLED STEEL IN WATER SO AS TO TRANSFORM THE SURFACE LAYERS OF THE STEEL SUBSTANTIALLY COMPLETELY INTO MATENSITE, (D) INTERRUPTING THE WATER QUENCHING AFTE A TIME INTERVAL SUCH THAT THE CORE OF THE STEEL IS AT A TEMPERATURE HIGHER THAN THE URFACE LAYERS AND IS TRANSFORMED ONLY PARTIALLY INTO MATENSSITE SO AS SUBSTANTIALLY TO COMPRISE AUSTENITE AND MATERNSITE. (E) THEREUPON CONTINUING THE COOLING OF THE STEEL AT A RATE SLOER THAN THAT OF SAID WATER QUENCHING SO AS TO EVEN UP THE TEMPERATURE DIFFERENCE BETWEEN THE SURFACE LAYERS AND THE CORE, AND (F) THUS EFFECT LIGHT TEMPERING OF THE SURFACE LAYERS AT A TEMPERATURE NOT BELOW 150*C. WITH CONTINUED TRANSFORMATION OF THE REMAINING AUSTENITE IN THE CORE INTO MARTENSITE TO AN EXTENT SUCH AS TO PRODUCE A MAINLY LIGHTLY TEMPERED MARTENISTIC STRUCTURE THROUGHOUT THE SECTION.

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