US3470037A

US3470037A - Method of treating alloy steel

Info

Publication number: US3470037A
Application number: US562720A
Authority: US
Inventors: Kazuhisa Suzuki
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1965-07-24
Filing date: 1966-07-05
Publication date: 1969-09-30
Anticipated expiration: 1986-09-30
Also published as: DE1508424B1

Description

Sept. 30, 1969 Filed July 5. 1966 CHARPY IMPACT VALUE AT-50 "C,kg-m/;m CHARPY IMPACT VALUE AT-f':v0C,kgm/cm KAZUHISA SUZUKI METHOD OF TREATING ALLOY STEEL 4 Sheets-Sheet 1 o l x l I 1 0 200 400 600 800 I000 I200 I400 TEMPERATURE,C

o l 1 l 1 TIME, SECONDS TEMPERATURE, C

KAZUHISA SUZUKI I 3,470,037

METHOD OF TREATING ALLOY STEEL Sept. 30, 1969 Filed July 5, 1966 4 Sheets-$heet 2 I I l COOLING TIME, SECONDS TEMPERATURE, C

COOLING TIME, SECONDS TEMPERATURE, C

P 30, .1969 KAZUHISA SUZUKI 3,470,037

METHOD OF TREATING ALLOY STEEL Filed y 1 4 Sheets$heet :s

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a. a: I U o l l l A 1 1 0 2O [4O 60 GO I00 I20 TIME, SECONDS United States Patent US. Cl. 148127 Claims ABSTRACT OF THE DISCLOSURE The steel is heated to a temperature above the A transformation point and is then cooled to approximately the temperature at which martensitic transformation of the steel begins, along a cooling curve approximating the cooling curve resulting from the welding thermal cycle, after which the steel is cooled more slowly.

This invention relates to a method of treating alloy steel.

The conventional manufacture of alloy steels of high strength includes a heat treatment comprising quenching 'ice alloy steel to the temperature to which the steel is subsequently heated, for example, by a welding operation;

FIG. 2 is a plot relating the notch toughness of the heat affected zone of the same alloy steel as of FIG. 1 to the time interval in which the steel is cooled from a temperature of 800 C. to 500 0.;

FIG. 3 is a plot relating the cooling of another alloy steel to the phases thereof;

FIG. 4 is a plot indicating various cooling rates of the steel of FIG. 3 after it has been subjected to submerged arc welding;

FIG. 5 is a plot superimposing upon the plot of FIG. 3 a portion of the band included by the extreme members of the set of curves of FIG. 4;

FIG. 6 is a plot comparing the notch toughness of conventional alloy steel to another alloy steel treated according to the present invention, at various temperatures;

FIG. 7 is a plot superimposing upon the FIG. 1 plot a like plot for the alloy steel of FIG. 6, and

FIG. 8 is a plot superimposing upon the plot of FIG. 2 a like plot for the alloy steel of FIG. 6.

Examples of two conventional high strength alloy steels are set forth in Table 1.

TABLE 1 Type Heat treating C Si Mn P S Ni Or A 870 0. WQ, 050 C. no-.-" 16 .38 .98 .01 .016 .95 .47 B 870 C. WQ, 650 0. AG... .15 .32 1.05 .007 .018 .93 .56

Charpy im- Yield Tensile Elon- Reduction pact value point strength gation of area, 50 C. Type Mo B (kg/mm?) (kg/mm?) (percent) (percent) (kgmJcmfi) the hot rolled ingot. The purpose of the heat treatment is Under the heat treating column is indicated a conto increase the tensile strength and toughness of the alloy ventional heat treatment. When these conventionally steel. The alloy steels are steels containing one or more alloying elements. Typical alloying elements are silicon, manganese, nickel, chromium, molybdenum, boron and the like. When the alloying elements constitute about 2% or more by Weight of the steel the steel when conventionally heat treated has a tensile strength on the order of 70 to 100 kg./mm. and is known as a high strength alloy steel. However, in spite of their great initial toughness it is found that when these high strength alloy steels are subjected to welding, particularly high heat input welding such as automatic welding, the heat of the welding severely decreases the notch toughness of the steel.

It is an object of the present invention to provide a method of treating alloy steel, especially, but not only, high strength alloy steel, for the purpose of substantially mitigating or essentially eliminating a decrease therein of toughness upon subjection to a welding operation.

Briefly, the method of the present invention involves a heating of the steel followed by a cooling thereof at a rate approximating the rate at which the steel will be cooled after the welding operation.

Other objects and features of the present invention will become apparent from the following detailed description and accompanying drawings, in which drawings:

FIG. 1 is a plot relating the notch toughness of the heat affected zone of a conventionally treated high strength treated alloy steels are welded it is found that there is a subsantial decrease in their notch toughness in areas thereof which have been heated by the welding operation, especially areas which have been heated to temperatures at which substantial austenite formation takes place. Thus, for example, when the type B steel of Table 1 is heated to temperatures up to 1350 C., as would occur in the case of welding, and during subsequent cooling is allowed to cool from 800 C. to 500 C. during a period of about 25 to 28 seconds a substantial decrease in the notch toughness thereof, as indicated by the Charpy impact value, occurs (FIG. 1). Thus, the Charpy impact value of the area of the type B steel which has been heated to a temperature of 1350 C. has been lowered to 1 kg. m./cm. from an initial value for the type B steel of 7.6 kg. m./ cm.

Furthermore, the amount of decrease in the Charpy impact value increases up to a certain point as the time interval for cooling from 800 C. (i.e., about the A transformation temperature) to 500 C. (i.e., about a temperature at which transition from austenite to martensite begins) is increased. Thus, for example, in the case of the type B steel the Charpy impact value rapidly decreases as the time interval for cooling from 800 C. to 500 C. is increased up to about 40 seconds (FIG. 2). Such time intervals for cooling from 800 C. to 500 C. are not uncommon after welding.

In the prior art it was attempted to prevent or mitigate this decrease in notch toughness by subjecting the steel to a preliminary heat treatment as indicated in Table 1 or to a post-welding heat treatment. As indicated above, a severe decrease in the notch toughness of the steel upon welding cannot be eliminated by the preliminary heat treatments of the prior art. Post-welding heat treatments also have not been particularly successful. According to the present invention, it has been found that by subjecting the steel to a preliminary cooling cycle approximating the cooling cycle which the steel will be subjected to when it is welded and subsequently cooled essentially eliminates any significant decrease in the notch toughness of the steel upon welding and cooling. Furthermore, it has been .found that the alloy steel may have a relatively low alloy content and, nevertheless, be susceptible to the treatment of the invention.

An example of a steel having a tensile strength of 70 kg./mm. but, nevertheless, having a relatively low alloy element content is one containing by weight 0.14% C, 0.25% Si, 1.27% Mn, 0.53% Ni, 0.24% Cr, and 0.19% Mo. In the phase diagram of this steel (FIG. 3), I indicates the austenite zone, II indicates the ferrite zone, III indicates the pearlite zone, IV indicates the transitional zone between pearlite and martensite, and V indicates the martensite zone. The steel has an A transformation point of approximately 710 C. and an A transformation point of approximately 810 C. According to the invention, the steel is heated to a temperature immediately above the A transformation point and starting from a time designated zero is cooled as indicated by a cooling curve I. The steel is rather rapidly cooled to the initial point of martensite transformation M (about 400 C.) and, thereafter, is continuously cooled down through the termination point of martensite transformation M (about 220 C.) and down to room temperature. A rate of cooling between the A transformation point and the starting point of martensite transformation M is provided which will approximate the post-welding cooling rate between those temperatures.

Four samples of the steel which have been cooled from a temperature immediately above the A transformation temperature at a rate described by curve 1 (FIG. 3) are each subjected to a submerged arc welding operation wherein they are rapidly heated to a temperature of 1350 C. and subsequently cooled each at a different rate. The samples are designated A, B, C and D, respectively, and the time at which cooling begins is considered the zero time (FIG. 4). The cooling curves for the four samples are superimposed as a band upon the phase diagram of the steel (FIG. 5). The line a-a' corresponds to the cooling curve for sample A and the line dd corresponds to the cooling curve for sample D. The area between line 11-11 and line d-d' is illustrated as a hatched band to represent any number of cooling curves, such as that for sample B and that for sample C, therebetween. It is noted that in such an area a cooling time from the A transformation point to 500 C. range from 5 to 50 seconds while a cooling time from 500 to 200 C. range from to 4,000 seconds. Thus, it is seen that, according to the invention, cooling from a temperature about the A transformation temperature after the welding operation is conducted at a rate approximating the rate of cooling from a temperature about the A transformation temperature before the welding operation. This is readily appreciated by noting the similarity in shape between cooling curve 1 (FIG. 3) and the area defined by the cooling curves for samples A and D (FIG. 5). The term rate is employed herein to designate the continuum of rates which can best be illustrated by a cooling curve.

It is essential to the invention that cooling between the A transformation temperature and the temperature at which transformation to martensite begins (M be ap' proximately the same in the preliminary treating step (FIG. 3) as in the cooling subsequent to welding (FIG. 5). Furthermore, it is preferred for the best results that in both the preliminary treatment and subsequent to welding the steel be cooled from about the temperature at which transition from austenite to martensite begins toward room temperature at rates approximately the same, and less than the rates of cooling used in cooling from the temperature A to temperature M If desired, after a preliminary treatment according to the invention and before the welding operation, the duc tility of the steel may be increased by the formation of spheroidic carbide by means of tempering the steel at temperatures from about 150 C. to immediately below the A transformation temperature, the higher temperatures in this range affording greater degrees of ductility.

The type B steel preliminarily treated and cooled subsequent to welding according to the present invention demonstrates little of the conventional embrittlement caused by welding. This is illustrated by the fact that the Charpy impact value of the steel after the Welding operation is still high.

Particularly good results are obtained according to the present invention when the alloy steel is fairly high in its content of alloy elements, i.e., at least 3 or 4% by weight. In conventional steel terminology, carbon is not considered an alloy element because its presence is required to have any steel at all. An example of a rather high alloy steel is the following one, of which the composition is indicated in terms of percentage by weight in Table 2 and the properties of which are indicated in Table 3.

The steel of Tables 2 and 3 is subjected to a preliminary treatment according to the invention like that illustrated in FIG. 3 with respect to another steel. The Charpy impact value of this preliminarily treated steel according to the present invention and of the conventionally treated type B steel are measured at temperatures between about C. and +20 C. It is found that a curve In which represents the Charpy impact values for the preliminarily treated steel according to the present invention is at approximately the same level as a curve h which represents the Charpy impact values for the type B steel (FIG. 6), whereby it is demonstrated that preliminarily treated steel according to the present invention has a toughness comparable to the toughness of conventionally treated steel of the prior art.

Then the steel of Tables 2 and 3 is subjected to a welding operation in which a portion of the steel is heated to a temperature as high as 1350 C. After the welding operation this steel is cooled in accordance with the invention at the same rate starting at the A transformation temperature as illustrated in FIG. 5 for another steel. Then the Charpy impact values of areas of this steel which have been heated by the welding operation to various temperatures up to 1350 C. is compared with the Charpy impact values of like areas of the conventionally treated type B steel which has been subjected to a like welding operation. The Charpy impact values for the conventionally treated type B steel which have been illustrated in FIG. 1 are illustrated again in FIG. 7 wherein the curve representing them is designated curve 1'. A curve i, which represents the Charpy impact values of the steel of Tables 2 and 3 which has been preliminarily treated and subjected to welding followed by cooling according to the present invention, does not exhibit the very pronounced downturns exhibited by the curve 1'. Thus, the steel treated and welded according to the present invention essentially lacks the embrittlement found in the conventionally heat treated and welded steel. Thus, according to the present invention, embrittlement of alloy steel upon welding, which has been particularly severe in areas of the steel heated to a maximum temperature within approximately 100 C. of the A transformation temperature or heated above the A transformation temperature (FIG. 1) is avoided.

The interval during which cooling from about 800 C. (i.e., about the A,- transformation temperature) to about 500 C. (i.e., about the beginning of the transition from austenite to martensite) is effected after the welding operation is found to be very significant with respect to the Charpy impact value of conventionally treated alloy steels. It is disclosed above that the Charpy impact value of the conventionally treated type B steel decreases very sharply as the time interval is increased to about 40 seconds and, then, finally levels oflf (FIG. 2). The cooling curve of FIG. 2 is reproduced as curve I in FIG. 8. A curve k, which represents the effect of the time interval upon the Charpy impact value of the steel of Tables 2 and 3 preliminarily treated and cooled after welding according to the present invention, indicates a gradual, rather than a sharp, decrease. This is another advantage of the present invention in that it permits more latitude in selecting rates of cooling.

In the preliminary treatment of the present invention, preferably the temperature of the steel is held immediately above the A transformation temperature until uniform austenite structure has formed throughout the steel. Generally, a convenient point for applying this treatment is after the blooming and rolling of the ingot. It is found that the preliminary treatment of the invention results in the ultimate formation mainly of bainite structure. Appatently, by virtue of the formation of the bainite structure it is possible to obtain alloy steels which resist embrittlement upon welding. Furthermore, frequently in the prior art an extremely high alloy steel was employed in order that the steel would still be reasonably tough even after being embrittled by the welding operation. However, such extremely high alloy steels tended to be unduly hard and, therefore, susceptible to stress corrosion cracking in the presence of sulfides. Thus, this problem would arise, for example, in Welded steel vessels for propane gas since propane gas contains sulfides. This problem is entirely avoided by the present invention.

It Will be appreciated that this invention is particularly useful in the case of automatic welding such as submerged arc welding and carbon dioxide arc welding, because in such welding the heat input to the steel is particularly great and, therefore, the embrittlement problem is particularly severe.

The method of the present invention is particularly useful in any context in which it is desired to automatically weld high strength, i.e., alloy, steel, such as shipbuilding, and the construction of bridges, pressure vessels, tanks, rocket chambers, and the like.

The invention is not to be construed as limited to the particular embodiments disclosed herein, since these are to be regarded as illustrative rather than restrictive.

What I claim and desire to secure by Letters Patent is:

1. A method of pre-treating and welding alloy steel comprising heating the steel to a temperature immediately above the A transformation temperature of the steel, then cooling the steel at a first cooling rate to approximately a temperature at which martensitic transformation of the steel begins, and finally cooling the steel to room temperature, then subjecting the steel to a welding operation, wherein the portion of steel heated by the welding operation is heated to a temperature above about the A transformation temperature, cooling said portion of steel after the welding operation at a rate substantially the same as said, first cooling rate between approximately the A transformation temperature and approximately a temperature at which martensitic transformation begins, and then cooling said portion of steel to room temperature at a slower rate whereby embrittlement of the steel by the welding operation is essentially avoided.

2. A method according to claim 1, in which after both of said cooling steps carried out at said first cooling rate, the steel is cooled from approximately a temperature at which transition from austenite to martensite begins toward room temperature at rates approximately the same, and less than the rates of cooling used to cool the steel from said A transformation temperature to the beginning of said martensitic transformation.

3. A method of heat treating alloy steel which comprises the steps of heating the steel above the A transformation temperature of the steel, cooling the steel from said A transformation temperature to about 500 C. for a period of time ranging from about 5 to 50 seconds, and then cooling the steel from about 500 to 200 C. for a period of time ranging from about 20 to 4,000 seconds and finally cooling the steel to room temperature, then subjecting said steel to welding whereby the portion of steel heated by the welding operation is heated to a temperature above about the A transformation temperature and then cooling said portion of steel to room temperature, said first and second cooling steps prior to said welding substantially following the cooling curve followed by the resultant weld, whereby embrittlement of the steel is substantially eliminated.

4. A method as claimed in claim 3 wherein after the steel has been subjected to the cooling treatment as defined in claim 3, the steel is tempered. at temperatures from about C. to below the A transformation temperature.

5. A method as claimed in claim 3 wherein the steel is transformed from the austenitic into the bainitic structure.

References Cited UNITED STATES PATENTS 2,224,998 12/1940 Wood et al. 148-153 2,441,628 5/1948 Grifiiths et al. 148143 2,732,323 1/1956 Linnert 148127 3,103,065 9/1963 Rectenwald 148-127 X 3,111,436 11/1963 McGavin 148-143 3,192,079 6/1965 Takagi et al. 14834 X CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 29498