US3827923A - Case hardening super high speed steel - Google Patents

Abstract

A LOW CARBON, CASE HARDENED HIGH SPEED STEEL COMPOSITION AND ARTICLE HAVING A SURFACE HARDNESS IN EXCESS OF 70 ROCKWELL C IS PROVIDED CONSISTINF ESSENTIALLY OF: CARBON, 10 UP TO ABOUT .30% CHROMIUM,ABOUT 4% MOLYBDENUM, ABOUT 3-10% TUNGSTEN ABOUT,11 2-7% VANADIUM ABOUT, 1-3% COBALT ABOUT, 4-12%

THE BALANCE IS SUBSTANTIALLY IRON AND CARBURIZED AT 1725* F., HARDENED AND TEMPERED.

Description

1974 R. F- HARVEY ET OASE HARDENING SUPER HIGH SPEED STEEL 1 Filed April 21. 1972 F'n c1.

Aus-rezNn-lc GRAIN SIZE 25.0

CARBURIZED AND HARDENED CAsE N IT'AI. E-rcH IQOOX United States Patent 3,827,923 CASE HARDENING SUPER HIGH SPEED STEEL Richard F. Harvey, Orchard Lake, and George E. Moore, Farmingtou, Mich., assignors to Sun Steel Treating, Inc.

Continuation-impart of abandoned application Ser. No.

844,580, July 24, 1969. This application Apr. 21, 1972,

Ser. No. 246,149

Int. Cl. C22c 35/24 US. Cl. 148-315 Claims ABSTRACT OF THE DISCLOSURE A low carbon, case hardened high speed steel composition and article having a surface hardness in excess of 70 Rockwell C is provided consisting essentially of:

Carbon, .10 up to about .30%

Chromium, about 4% Molybdenum, about 3-10% Tungsten about, 1' /2-7% Vanadium about, 1-3% Cobalt about, 4-12% The balance is substantially iron and carburized at 1725 F., hardened and tempered.

This application is a continuation-in-part of our copending application Ser. No. 844,580, filed July 24, 1969, now abandoned.

This invention relates to high speed steel articles and compositions and particularly to a new and improved class of high speed steels which have higher hardness, finer grain size and improved physical properties than conventionally available steels.

The alloys of the present invention are designed to be an improvement over the M40 or Superhard high speed steels. As is well known the M40 or superhard high speed steels are basically high carbon, high cobalt modifications which were developed as an improvement over existing grades for improved hardness, red hardnes and cutting ability. Insofar as it is known this important class of high speed steel as exemplified by A.I.S.I. M41, M42, M43, M44, M46, and M47 was first developed and disclosed by Richard F. Harvey and coworkers.

In this connection the first printed announcement of this class of steels appeared in Metal Progress, May 1960, p. 113. Research efforts on the high carbon high cobalt grades culminated in the issuance of Us. Pat. 3,113,862, on Dec. 10, 1963 to Richard F. Harvey and Charles W. Schuck. An excellent article on the superhard high speed steels appeared in the Tool and Manufacturing Engineer on July 1966, pp. 60 and 61. This article, entitled Superhard Tool Steels was written by Dr. S. G. Fletcher and C. R. Wendell of the Latrobe Steel Company.

In view of the foregoing an object of our present invention is to provide a new family or class of super high speed steels and articles characterized by higher hardness, finer grain size, and better physical properties than the conventionally available high speed steels.

Another object of this invention is to provide a relatively low carbon analysis which when case hardened results in the development of residual surface compressive stress for greater strength and load carrying capacity.

Another object of this invention is to provide a new class of low carbon, super high speed steels which when case hardened results in higher red hardness than as compared to conventionally available analyses.

Other objects and many of the advantages of our invention will be readily appreciated and will be better understood by reference to the following description of the ice new class of steels together with their important, unique advantages.

For decades it has been an important aim of metallurgists and tool steel producers to make high speed steels with good combinations of hardness, red hardness, and toughness. The compositions of the present invention represent a distinct step forward in the progress of high speed steels to narrow the gap between high speed steels and cemented carbides.

We have discovered that the new steels having compositions except for carbon which correspond to the compositions of the superhard or M40 series of high speed steel, posses greater hardness, red hardness and ductility when case hardnened according to the principles of the invention.

When proceeding according to our invention, a super high speed steel composition is preselected as having fairly good combinations of hardness, red hardness and ductility. These steels are then modified by a drastic reduction in the carbon to up to about .25% carbon. In this condition the steels are hardenable only up to about 42 Rockwell C. However on carburizing to a case carbon of about 1.20% and a core carbon of about .25 followed by heat treating, very exceptional properties are obtained. We have obtained peak hardness of Rockwell C /2 at a tempering temperature of 1050" F. This is about 100 F. higher than the peak secondary hardness obtained on tempering the conventional M40 series of high speed steels.

Furthermore low carbon high speed steel made by the teachings of this invention exhibit very fine austenitic grain size in the hardness and untempered condition. Other factors which contribute significantly to greater toughness is the low carbon, tough core and the high degree of compressive stress introduced in the surface of the heat treated high speed steel.

The chemical analysis of our steel is such that the carbon content has been reduced to less than one quarter of the carbon present in the conventional M40 series high speed steels. Also we find .that in order to obtain the important exclusive advantages of this invention a balance must be maintained between the austenite forming elements and the ferrite forming elements. Furthermore the balance of the austenite forming elements and the ferrite forming elements in the case and in the core must be maintained within critical controlled limits to obtain desired results as will be explained more fully hereinafter.

Listed below in Table I are six super high speed steels of the so called M40 series.

TABLE I M40 series of super high speed steels C Cr V W Mo Co The corresponding low carbon compositions, tentatively designated as M60 series high speed steels of the present invention are tabulated herewith in Table H.

TABLE 11 M60 series of low carbon high speed steels C Cr Va W Mo Co Percent Carbon .26

Manganese .24 Silicon .52

Chromium 3.61 Vanadium 1.69

Tungsten 4.48 Molybdenum 5.83 Cobalt 5.92

The experimental heat was forged to a 1% inch square billet which was annealed.

A test ring 1%" OD by 1" ID was gas carburized in a pit type Leeds & Northrup Homo Carburizing furnace. The carburizing temperature was 1725 F. and the time was 8 hours using 3 cu. ft. of natural gas per hour in an RX Carrier Gas. The case depth was .060" and the surface carbon was 1.20%. After carburizing the test piece was cooled in air to room temperature followed by annealing at 1600' F.

The test piece was austenitized at 2165 F. to an intercept grain size of 25.0. The photomicrograph FIG. 1 illustrates the fine austenitic grain size of 25.0. This is quite fine for austenitized high speed steel and is considerably finer than the corresponding M40 Series of high speed steel which exhibit austenitic grain sizes of about 13 to 16 in the hardened and untempered condition using equivalent austenitizing conditions.

On tempering the hardness of the steel made in accordance with this invention is as follows.

Tempered: Rockwell C hardness case 800 F.2 hrs. 60 850 F.2 hrs. 63 900 F.2 hrs. 64 950 F.2. hrs. 67 /2 1000 F.2 hrs. 69 /2 1050 F.2 hrs. 70 /2 1100 F.2 hrs. 69

It should be noted that peak secondary hardness was obtained with a tempering temperature of 1050 F. whereas steels of the M40 series exhibit maximum secondary hardness on tempering at about 950 F.

In FIG. 2 curve 1 represents the hardness for an M42 steel after various tempering treatments. It Will be noted that the peak secondary hardness 2 is obtained with a relatively low tempering temperature of 950 F.

Curve 3 represents the hardness response to tempering of the improved steel of the present invention. The peak secondary hardness 4 is obtained with a tempering temperature of 1050" F.

This indicates greater red hardness and resistance to tempering of the steel of the present invention. An increase of 100 F. in the temperature for peak secondary hardness represents a significant increase in resistance to tempering which should be of interest in all elevated temperature applications where high speed steel is used.

We have found that it is essential that a balance be maintained between the austenite forming elements and the ferrite forming elements. This balance must be maintained between the case and the core for best results. The various elements vary in their alloying effect in accordance with the factors tabulated in Table H.

4 TABLE H Relative Effect of Alloying Elements Austenite forming:

Carbon 35 Nitrogen 35 Cobalt l Ferrite forming:

Chromium 1 Silicon 5 Molybdenum 2 Tungsten 1 Manganese 1 Vanadium 3 In applying these factors, the following tabulation will illustrate the ratio of the austenite forming elements to the ferrite forming elements which has been found to result in optimum hardness, red hardness, and fine grain size which combination is an outstanding characteristic of the present invention.

Table III illustrate the application of these factors for the core of the example of the steel of the present invention and Table IV represents the application of these factors for the case of the steel of the present invention.

TABLE III Austenite forming elements Ferrite forming elements Carbon .26X35=9.10 Chromium. Cobalt 5. 92Xl=5. 92 Silicon Manganese. .24 1=.24 Total austenite Molybdenum.-..- 5. 83X2=1L 66 Alloying effect... 15. 02 Tugsten 4. 48X1=4. 48 Vanadium 1. 60 3=5. 07

Total ferrite alloying effect 27. 71

Total austenite alloying effect 15.02 Norm-Ratio Total ferrite alloying effect 27. 71

TABLE IV Austenite forming elements Ferrite forming elements Carbon 1. 20X35=42.0 Chromium 3. 61Xl=3. 61 Cobalt 5. 92X1=5. 92 Silicon .52X5=2.60 Manganese. .24X1=.24 Total austenite Tungstem. 4. 48X1=4. 48 alloying effect-.- 47. 92 Molybdenum. 5.83 2=11. 66 Vanadium 1 69X3=5 07 Total ferrite alloying effect 27. 71

Total austenite alloying effect 47. 92 NOEL-Ratio ----=--=1.73

Total ferrite alloying effect 27. 71

We find that the ratio of the total austenite alloying effect to the total ferrite alloying effect for the core in the example cited is .57 and preferably this ratio should be within the limits of about 0.4 to 0.8.

We find also that the ratio of the total austenite alloying effect to the total ferrite alloying effect for the case in the example cited is 1.73 and preferably this ratio should be within the limits of about 1.55 to 1.95.

It will be observed also that in the example cited the ratio of the total austenite alloying effect in the case of the total austenite alloying effect in the core is 47.92/ 15.02=3.l9. This is a critical ratio which should be rigidly held between the limits of about 2.90 and 3.50 for optimum results.

FIG. 3 illustrates the extent of the total austenite alloying effect in the case and core for optimum properties.

The area abcd represents the useful area of compositions including the case and core according to the teachings of the invention.

From the foregoing it will be noted that the improved class of high speed steels of the present invention are characterized by high hardness and resistance to softening along with a fine grain size and high ductility.

While we do not wish to be bound by the limitations of a theory we believe that the superior and unique results of this invention are due in large measure to the effect of residual compressive stresses developed during case hardening. These compressive stresses are believed to be beneficial in creating higher hardness, higher red hardness and a finer grain size.

Go the basis of the foregoing our improved composition for a case hardening super high speed steel lies within the following range.

The carbon in the case after case hardening should be in the range 1.10% to 1.30%, preferably about 1.20%.

While we have set out certain preferred practices and embodiments of our invention in the foregoing specification it will be understood that this invention may be otherwise embodied within the scope of the following claims.

We claim:

1. A high speed steel composition characterized by having a surface hardness in excess of 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.15 to 0.25% carbon; 0.45% maximum manganese; 0.45% maximum silicon; 3.5 to 4.5% chromium; 1.0 to 3.0% vanadium; 1.15 to 6.0% tungsten; 3.5 to 9.5% molybdenum; 5.0 to 8.0% cobalt, with the remainder essentially iron; said composition having been carburized to a surface carbon content of about 1.10 to 1.30%, hardened and tempered.

2. A high speed steel composition characterized by having a surface hardness in excess of 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.15 to 0.25 carbon; 0.45 maximum manganese; 0.45% maximum silicon; 3.5 to 4.5 chromium; 1.0 to 3.0% vanadium; 1.5 to 6.0% tungsten; 3.5 to 9.5 molybdenum; 5.0 to 8.0% cobalt, with the remainder essentially iron; said composition having been carburized at 1725 F. to a surface carbon content of about 1.20% hardened at 2165 F. and tempered at 1050 F.

3. A high speed steel composition characterized by having a surface hardness of over 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.10 to 0.30% carbon; 0.60% maximum manganese; 0.60% maximum silicon; 3.0 to 5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdneum; and 4.0 to 12.0% cobalt, with the remainder substantially iron; said composition having been carburized to form a surface hardened case having a ratio of total austenite alloying effect in the case to the total alloying elfect in the core of between about 2.90 to 3.50.

4. A high speed steel composition as claimed in claim 3, having been carburized at 1725 F. to a surface carbon content of about 1.2%, hardened at 2165 F. and tempered at 1050" F.

5. A case hardened high speed steel composition consisting of about 1.10% to 1.30% carbon in the case about 0.10 to 0.30% carbon in the core, characterized by having a surface hardness of above about 70 Rockwell C the balance of the composition consisting essentially of 3.0 to 5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdenum; and 4.0 to 12.0% cobalt; the remainder being substantially iron with residual elements in normal amounts; said composition having an intercept grain size in the case of about 25 having been hardened and tempered.

6. A case hardened high speed steel composition characterized by having a surface hardness in excess of 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.10 to 0.30% carbon; 3.50 to 4.50% chromium; 1.0 to 3.0% vanadium; 1.5 to 6.0% tungsten; 3.5 to 9.5% molybdenum and 5.0 to 8.0% cobalt; the remainder being substantially iron with residual elements in normal amounts; said composition being characterized by a ratio of total austenite alloying effect to total ferrite alloying effect of between about 0.4 to 0.8; and after carburizing being characterized by a ratio of total austenite alloying effect in the case to the total ferrite alloying effect in the case of between about 1.55 to 1.95; said composition having been hardened and tempered.

7. A high speed steel composition as claimed in claim 6 which has been hardened at 2165 F. and tempered at 1050 F.

8. A case hardened article of high speed steel characterized by having a surface hardness in excess of 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.15 to 0.25% carbon; 0.45% maximum manganese; 0.45% maximum silicon; 3.5 to 4.5% chromium; 1.0 to 3.0% vanadium; 1.5 to 6.0% tungsten; 3.5 to 9.5% molybdenum; 5.0 to 8.0% cobalt, with the remainder essentially iron; said composition having been carburized to a surface carbon content of about 1.10 to 1.30% hardened and tempered.

9. A low carbon case hardened article characterized by having a surface hardness of over 70 Rockwell C and a low carbon high strength core consisting essentially of about 0.10 to 0.30% carbon; 0.60% maximum manganese; 0.60% maximum silicon; 3.0 to 5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdenum; and 4.0 to 12.0% cobalt, with the remainder substantially iron; said composition having a ratio of total austenite alloying effect in the case to the total alloying effect in the core of between about 2.90 to 3.50 and having been carburized at 1725 F. to a surface carbon of about 1.20%, hardened at 2165 F. and tempered at 1050" F.

10. A case hardened article consisting of about 1.20% carbon in the case, and about 0.10 and 0.30% carbon in the core, characterized by having a surface hardness of above about 70 Rockwell C on tempering at 1050 F., the balance of the composition consisting essentially of 3.0 to 5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdenum; and 4.0 to 12.0% cobalt; the remainder being substantially iron with residual elements in normal amounts; said composition having an intercept grain size in the case of about 25 on austenitizing at 2165 F. and tempered at 1050 F.

References Cited UNITED STATES PATENTS 1,991,805 2/1935 Kluger -126 H 2,033,927 3/ 1936 De Fries 148-315 2,081,394 5/ 1937 De GOlyer 75-126 H 2,147,122 2/ 1939 Emmons 75-126 H 2,289,449 7/ 1942 Nelson 75-126 H 2,676,098 4/1954 Payson 148-39 3,259,489 7/ 1966 Hamaker 148-31 HYLAND BIZOT, Primary Examiner

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