US2332441A - Carburized article - Google Patents

Description

Patented is, 194; s

2,332,141 I osnnunrzan narrow,

Martin Fleischmann, (Danton,v Ohio, assignor to The Tlmken Roller Bearing Compa y. Canton,

Ohio, a corporation of Ohio in Drawing. Application April 4, 1941,

I Serial No. 386,847

'7Claims.

This invention relates to case carburized steel articles, and to steels for making them.

Plain carbon steels have been used extensively for the production of carburized, or case hardened, articles. However, the core properties of carburized plain carbon steels are inadequate for many purposes, for which reason alloy steels have been used widely in the manufacture of such articles, especially those subjected to stress such, for example, as bearings and gears; It has been considered that nickel is desirable in producing cases that are satisfactorily resistant to spalling and related phenomena, so that apart from plain to standard commercial practices and other chromium steels free, or substantially free, from nickel develop excessive amounts of carbides in n the case, particularly at the'extreme outer surface, which causes the cases tojbe objectionably brittle. the surface of acarburized case contain from about 0.5 to about l-or at most 1.1 percent of carbon, because with higher surface concentrations of carbon the casetends to bev brittle and to spall in service. Experience has shown that when steels such as S. A. E. 4120 (proposed) and 5120 are carburized and hardened the surface carbon steels the bulk of the carburized articles have been produced from steels containing nickel with or without such other carbide-forming eleq ments as chromium and molybdenum. Examples of such nickel-containing carburizing steels are S. A. E2315, 3115, 3120, 3215, 3312, 4320, 4615 and 4815.

The existence of a nickel shortage, such as is imminent at present,- would make it necessary to give up, at least for many purposes, the standard nickel-containing carburizing steels, which would work a hardship upon various industries unless steels of equally satisfactory carburizing characteristics and core properties were available to take their place. It is additionally important that such substitute steels be capable of being carburized in the same manner as the nickelconcentration of carbon is substantially in excess of that desired and thecases tend to-be excessively brittle, and this is characteristic of the nickel-free carburizing steels containing chromium. The same difliculty is encountered also in steels containing nickel together with chromlum orsimilar carbide-forming element in an amount such that carburizing produces a substantial amount of carbides at the surface of the case, for instance a steel containing about 4 per cent of nickel-and 1.5 per cent of chromium. This containing steels which are preferred at present because the carburizing treatments usedfor the latter class of steels have been carefully standardized by the various producers, and the necessity of going to different operating conditions would be objectionable from an operating standpoint. I

It has been proposed heretofore to use chromium steels for the production of carburized parts, and it is'known that satisfactory core properties can be produced with such steels. For instance, it has been suggested that carburized parts be made from a nickel-free chromium steel corresponding to what would-be S. 'A. E. 4120,

' containing about 0.15 to 0.25 per cent of carbon; about 0.5 to 0.8 per cent of chromium, about 0.5 to 0.8 per cent of manganese, and about 0.15-

characteristic of the chromium carburizing steels, of forming excessive amounts of carbides at the case surface when carburized, has prevented their general adoption for the making of case hardened Parts.

It is among the objects of thisinvention to provide chromium alloy steels for making carburized articles, which provide physical and mechanical properties adequate for the useslto which carburized steels are put, which may be carburized to provide cases of satisfactory depth and properties, and which may be satisfactorily case hardened by existing commercial practices.

A further object is to provide case hardened steel articles formed from steels containing chromium or other carbide-forming element in an amount such that upon case hardening there will normally be produced objectionable amounts of carbides at the surface of the case, in which th to 0.25 per cent of molybdenum. The mechanical properties of such a steel approach closely those of such popular nickel-containing carburizing steels as S. A. E. 3115 and 4615, which would indicate that such a nickel-free chromium steel should be capable of replacing the nickel hearing I steels for use in carburized parts. Experience has shown, however, that when carburized according formation of excessive amounts of surface carbides inthe case is repressed, and which provide ease and core properties adequate for the uses to which carburized and hardened steel articles are put.

A special object'is to provide'nickel-free alloy steels containing chromium which may be carburized and hardened to provide cases that are substantially free from excess carbides, in which the surface carbon is within the limits considered desirable for case hardened articles, and which are adequately. hard andwear resistant,

It is desired in general practice thatg which embody satisfactory core properties, which produce carburized cases that are satisfactorily resistant to spalling, and which may be made by the processes applied to nickel-containing steels.

Yet another objectis to provide carburized articles formed froii'i;nickel-free chromium steels whose case hardened surfaces are resistant to I spalling and are comparable in behavior t the cases produced by case hardening nickel-containing steels.

'Still another object is to provide a process of controlling the surface concentration of carbides in case hardening steel articles.

The invention is predicated upon my discovery that steels of carburizing grade which contain chromium in an amount such as to cause the production of objectionable amounts of carbides in hardened 'cases may be satisfactorily case hardened with avoidance of the difficultie encountered heretofore in their use for such purposes by substantially increasing the silicon content over the amount normal in such steels. Hereto fore silicon has been kept low in the carburizing steels, as is evidenced by the fact that the S. A. E.

specifications for carburizing steels provide that the maximum silicon content shall be 0.3 per cent; This restriction has apparently been founded upon the belief that an amount of silicon above that normally present in such steels will cause graphitization of a carburized case. strikingly enough, however, I have found that not only does silicon not cause difficulty in chromium steels of carburizing grades. but that actually it is beneficial in that it causes hardened case to be free from excess carbides at the case surface and to be comparablein properties to those of the'nickel-containing steels which are free from or of low chromium content. Thus through the practice of the invention it is possible to produce satisfactory case hardened articles from the chromium steels which have heretofore been unsatisfactory for this purpose because of excessive amounts of carbides in the surface of the hardened cases. Consequently. users of carburized parts need not be affected by a nickel shortage, and larger amounts of nickel may be made available for national defense pur- P ses.

The invention is applicable generally to carburizing steels containing chromium in an amount such as normally to cause the production of excessive amounts of carbides in hardened cases, and from what has been said it will be understood that it is applicable not only to nickelfree steels but also to other chrome steels that a are subject to the same trouble when case hardened, such for example, as nickel or other steels containing appreciable amounts of chromium. As applied to most purposes, relatively low contents of chromium suflice, say from about 0.4 or 0.5 per cent to about 3 per cent. The carbon content of the steels used in the practice of the invention will generally conform to that considered desirable for carburizing purposes, say about 0.1 to 0.3 per cent, or even 0.15 to 0.25 per cent, although as known in the art such steels may for special purposes, as to increase core strength, contain somewhat larger amounts of carbon, say up to about 0.4 or 0.5 per cent.

For the purposes of the invention the silicon content should be above that normal to such steels, i. e., more than about 0.3 per cent and. at least for most purposes, not over about 1.5 per cent. I now believe that the most satisfactoryresults are to be had with steels containing from about 0.5 to 1 per cent of silicon, which range is preferred for most purposes.

Together with iron and the impurities gener ally present in such steels, these constitute the essential alloying elements of the steels provided by and used in the practice of the present invention. The steels may contain, however, other elements in amounts which do not affect adversely the characteristics described herein. For example, they may contain other carbide-forming elements in addition to chromium, such as molybdenum and vanadium, either or both, which are commonly used in present-day carburizing steels. For instance, the steels may contain from about 0.1 to 0.5 per cent of molybdenum or from about 0.1 to 0.25 per cent of vanadium.

The invention may be described further with reference to pack carburizing because of its extensive commercial use. Thus, the article are formed from chromium steels containing abnormal amounts of silicon for such purposes, as detailed above, and are then carburized in pots according to standard or desired practice, after which they are hardened by heat treatment in the manner known to all metallurgists skilled in the art. For instance, pack carburized articles made from the chromiumsteels of increased silicon content described herein may b case hardened satisfactorily by applying the carburizing and hardening practices that are now satisfactorily used in commercial production with the nickel-containing steels referred to hereinabove. In this manner there are produced hardened cases which, as compared with thos produced by the pack hardening of the nickel steels low in or free from chromium, are of equivalent depth, mechanical and physical properties, and of satisfactory core strength and ductility, or toughness, and which are free from massive carbides at the surface. If the same steels with normal silicon content, say not over about 0.3 per cent, are carburized in the same manner the case will contain massive carbides and be brittle.

The invention may be understood further with reference to actual examples of its application to nickel-free steels because they demonstrate its applicability to this class of steels and the ability to thus effect a saving of nickel, which is important at present. For example, in one instance nickel-free steels containing about 1 per cent of chromium and varying amounts of silicon were pack hardened by the procedure used in pack carburizing nickel-containing steels. The steels were of the following composition:

Steel Av represents a nickel-free chromium steel which when pack carburized produces a case containing excessive amounts of carbides at the surface and which is brittle and liable to spall in use; its silicon content is that normal to carburizing steels. Steels B and C are representative, as to silicon content, of the chromium steels used 0 in the practice of thi invention. Hot rolled 1- inch round bars of the three steels were normalized at 1700 F. following which they were machined to provide a smooth and clean surface. They were then pack carburized and pot cooled. The bars were then cleaned and 0.0025 inch cuts were taken from the surfaces and analyzed for carbon, which showed the following results:

Carbon concentration at surface after pot carburization at- 1650 F. 1700 F. 1750 F.

1.60% C 1.05% C 0.89% C As will be observed from the foregoing table, steel A, containing normal silicon, produced in all instances cases of very high surface concentration offcarbon, actually substantially in ex containing substantially increased silicon 0011-.

tent, produced inall instances cases whose surface concentration of carbon-ls within the range considered in the trade to be desirable for carburized articles which are subjected to stress, name- I 1 between about 0.8 to 1.1 per cent.

Microscopic examination of the carburizedbars showed that the case of steel A as pot cooled after ,carburizing consisted at and adjacent the surface almost entirely of massive carbides, and

that after hardening at 1500 F. there still remained substantial amounts of massive carbide. Examination of steel B as pot cooled showed relatively little free carbide in the case, and that after hardening at 1500 F. such carbides as were initially present had been almost entirely put into solution. In the case of steel no carbides were observable in the case as pot cooled or after hardening at 1500 F. No graphite could be observed in any of the cases. I'he micro-structures of the carburized articles showed that because of the excessive content of massive carbides the case on steel A would be excessively brittle while in comparison the cases produced on steels B and C would be satisfactorily resistant to spalling and similar to cases produced on standard nickel carburizing steels.

The carburized specimen were fractured, and examination of the fractures showed that in comparison with steel A the increased amounts of silicon present in steels B and C not only had not adversely affected the case depth but also that the core structure was actually finer, which is desirable for some purposes.

As evidencing further the fact that the use of silicon in accordance with this invention is bene ficial to -the core properties, the following table 'gives the mechanical properties of bars of steels A and B which'had been. put through the carburizing cycle while packed in cast iron chips to avoid both carburization and decarburizatlon. The data show that actually the use of silicon (steel 13) increases the ultimate strength without seriously aifecting the ductility, or toughness, as-

compared with steels of the same composition I have observed also that chromium steels 0on taining silicon in accordance with the invention can be tempered at temperatures substantially higher, for a desired hardness level, than the same steels of normal silicon content, which is important for some purposes and thus extends the applicability of the present invention.

I have discovered further that the concentration of carbides at the surface of a carburized case may be reduced by increasing the carburiz- .ing temperature, or, if desired, the concentration may be increased by decreasing the carburizing temperature. This is contrary to the prevailing belief according to which the carburiz ing temperature would be reduced to reduce the surface concentration of carbide. otherwise, I have discovered that if pack car-' burizing at a given temperature produces a surface concentration of carbides that is undesirable. for a particular purpose, the concentration of 1 carbides may be lowered by carrying out the carburizing' at a higher temperature, andvice versa. This may be exemplified by reference to the data given in the foregoing table. Although the invention has been exemplified with reference to its application to nickelfree steels and to pack carburizing, it will be understood that it is not restricted thereto.- Thus, it is applicable equally to other chromium steels, such as those containing nickel, which have given difficulties because of the development .of objectionable amounts of massive carbides at the surface of the case. And by experience I have found that the same characteristic results attend'the use of standard gas carburizing procedures which do not include a diffusion step as part of the carburizing cycle. Reduction of surface concentration of carbon by the use of silicon in accordance with the 11'1- vention attends the use of such a cycle also, but ordinarily the diffusion step will itself bring the surface carbon concentration to a useful value 'so that the benefits-of the invention are most prominent and important in the case of pack hardening and gas carburlzing without a diffu- According to the provisions of the patent stat vutes, I have explained the principle and method of practicing my invention and have described what I now consider to represent its best embodimerit. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim; I l. A case carburized steel article formed from of carbon, from about 0.5 to about 3 per cent of chromium, about 0.5 to about 1 per cent of silicon, and substantially no nickel, and the remainder iron together with impurities and elements in amounts which do not adversely aflect the carburizing characteristics of the steel, and characterized by the carburized and hardened case being resistant to spalling and low in or substantially free from massive carbides.

2. A case carburized steel article formed from chromium carburizing steel containing as essential lalloying elements'irom about 0.1 to about 0.3 per cent of carbon, at least one carbideforming element including at least about 0.5

per cent'of chromium, the sum of said carbideforming elements beingnot over about 8.75"per cent, and more than about 0.5 to about 1.0 per with impurities and elements in amounts which Stated- 4- do not adversely affect the carburizing characteristics of the steel, and characterized by its carburized and hardened case being resistant to spalling, and low in or substantially free from massive carbides.

3. Chromium carburizing steel containing as essential alloying elements about 0.5 to about 3 per cent of chromium, about 0.5 to about 1 per cent of silicon, carbon from 0.11:0 about 0.3 per cent and in an amount such that the steel is hardenable by heat treatment, and the remainder iron together with impurities-and elements in amounts which do not adversely affect the carburizing and heat treating characteristics of the steel, and characterized by producing, upon being carburized and hardened by heat treatment, a carburized case resistant to spalling and low in or substantially free from massive carbides, and high tensile strength in the core as the result of heat treatment.

4. Chromium carburizing steel containing as essential alloying elements about 0.5 to about 1 per cent of chromium, about 0.5 to about 1 per cent of silicon, about 0.1 to about 0.5 per cent of molybdenum, carbon from 0.1 to about 0.3 per cent and in an amount such that the steel is hardenable by heat treatment, and the remainder iron together with impurities and elements in amounts which do not adversely affect the carburizing and heat treating characteristics of the steel, and characterized by producing, upon being carburized and hardened by heat treatment, a carburized case resistant to spalling and low in or substantially free from massive carbides, and high tensile strength in the core as the result of heat treatment.

5. Chromium carburizing steel containing as essential alloying elements about 0.5 to about 3 per cent, of chromium, about 0.5 to about 1 per cent of silicon, about 0.1 to 0.3 per cent of vanadium, carbon from 0.1 to about 0.3 per cent and in an amount such that the steel is hardenable by heat treatment, and the remainder iron together with impurities and elements in amounts which do not adversely afiect the carburizing and heat treating characteristics of the steel, and characterized by producing, upon being carburized and hardened by heat treatment, a carburized case resistant to spalling and low in or substantially free from massive carbides, and high tensile strength in the core as the result of heat treatment.

6. Nickel-free carburizing steel containing as essential alloying elements from about 0.1 to about 0.3 per centof carbon, about 0.5 to about 3 per cent of chromium, about 0.5 to about 1 per cent of silicon, and substantially no nickel, and the remainder iron together with impurities and elements in amountswhich do not adversely affect the carburizing characteristics of the steel, and characterized by producing, upon being carburized and hardened, a carburized case resistant to spalling and low in or substantially free from massive carbides. I

7. A case carburized steel article formed from chromium carburizing steel containing as essential alloying elements about 0.1 to about 0.3 per cent of carbon, about 0.5 to 3 per cent of chromium, about 0.5 to l per centoi silicon, and the remainder iron together with impurities and elements in amounts which do not adversely affect the carburizing characteristics of the steel, and characterized by the carburized and hardened case being resistant to spelling and low in or substantially free from massive carbides.

MARTIN FLEISCHMANN.