US2294412A - Stainless steel - Google Patents

Description

Patented; Sept. 1, 1942 I STAINLESS STEEL James R. Long, Annapolis, Md., assignor to Chicago Development Company, Chicago,

No Drawing. Application June 6, 1940, Serial No. 339,077

1 Claim.

My invention relates to stainless steels, particularly stainless steels employing manganese, and used in the austenitic state. Commercial stainless steels, in general, comprise iron, nickel and chromium. The type of stainless steel finding the greatest commercial applications is the so-called 18-8 steel which consists of 18% chm-- mium, 8% nickel and the remainder 'iron. This steel is truly austenitic, capable of being handled and worked in the cold state and having very great advantages in the fabrication of stainless steel machinery parts,' retorts, chemical reaction vessels and the like. The stainless steel of the cutlery type is, in general, not austenitic and my invention is not primarily concerned with this particular type of product.

Because of the scarcity and the relatively high cost of nickel and for other reasons, the replacement of nickel by some other metal has received special consideration, particularly in such countries as Germany; Manganese has been considered as a substitute for nickel and it has been found that manganese is only partially efiective in changing the body centered ferrite of chromium iron into face centered austenite. In such steels as have been produced employing manganese, it has been necessary to raise the carbon content and also to use an appreciably greater proportion of manganese to obtain the austenitic stainless steel properties found in 188 steel.

The relation between the carbon content and manganese necessary to produce a completely austenitic alloy is clear from the work of Bain, Davenport and Waring. The results obtained by these investigators are not fully conclusive because their work extended only up to 14% manganese at which percentage approximately 0.2% of carbon was found to be required to produce a completely austenitic alloy. By extrapolation it would appear from this work that approximately 20% of manganese would produce a completely austenitic alloy, but further work reported in The Metallurgist of February 26, 1937, shows that with 15% manganese and 20% of chromium 0.3% of carbon must be employed togive the desirable austenitic character to the alloy.

The increase of the carbon content in alloys of the type discussed has certain disadvantages. In such alloys as 18% chromium and manganese with sufficient carbon to insure completely austenitic properties, a hard brittle constituent is precipitated which has been tentatively identified as the B constituent of. Bain and Griffith. This compound is an iron chromium comtioned in pure alloys. Heycock reported an inability to find this compound in pure alloys. The use of increased carbon is also objectionable because increased carbon tends to cause suscepti- 'bility to inter-granular corrosion. Inhibitors such as columbium and titanium can not be used to inhibit such inter-granular corrosion, because such metals employed as inhibitors would completely extract the carbon from the manganese carbide.

The best outlook for the satisfactory production of high manganese austenitic stainless steels, in view of this situation, would, therefore, appear to'be through a considerable increase in the manganese content. Proceeding in this direction, however, has shown the tendency to produce the so-called epsilon phase which occurs in iron-manganese alloys containing 12 to 20% of manganese and very low carbon. The conditions for the stability of this epsilon phase as well as the conditions for the stability of the B phase of the iron-chromium system have not been definitely determined. Nevertheless, the very distinct presence of the two phases under conditions found in the pa'st has necessarily resulted in investigators proceeding in a somewhat different direction to produce true austenitic stainless steels of good grade.

Because of this situation, some attention has been directed towards the production of stainless steels which are only in part austenitic. It has been found that an alloy of 18% chromium, 8% manganese, 1% copper and 0.1% carbon when rapidly cooled from 1925 degrees F. will produce a structure which is approximately 60% austenite and 40% ferrite. Steels of this kind have received some attention and for some purposes have been considered to show some promise. Kinzel has stated that these duplex steels have a definite place in the industry since they are easier to cold work and machine than the austenitic alloys. Their cold drawing properties, however, are definitely inferior. An alloy of 18% chromium, 6% manganese, 4% nickel and 1% of copper has been found to be austenitic and is substantially equal to 18-8 steels in corrosion resistance and working properties.

One object of my invention is the provision of an improved austenitic stainless steel having as a pound, the stability of which is definitely quesconstituent thereof an appreciable to large quantity of manganese. I

Another object is the utilization of manganese in stainless steels in; place of nickel in such a manner as to assure austenitic properties with either no carbon or such a small amount of carbon as to have little or no deleterious effect on the steel such as identified hereinabove.

Another object is the improved use of manganese in stainless steels.

In the development of my invention I found that so-called pure manganese as it was delivered commercially to experimenters in general contained as much as 6% of impurities, generally iron, silicon, carbon, sulphur and aluminum. The iron content of manganese is not of primary significance when working on stainless steels, but the presence of significant quantities of silicon and aluminum, above 1 of the manganese, I consideredpossibly to be of significant interest. I'proceeded on the assumption that the silicon and aluminum content of manganese had to a considerable extent been overlooked by investi-' gators and that this might account to a substantial extent for the results obtained. To this end I was led to a consideration of substantially pure manganese produced by the. distillation of commercially pure manganese in vacuum or by the electrolysis of pure manganese sulphate solu- Methods for producing substantially pure tions. manganese have been given in the literature and need not be repeated here. I find that by the use of high purity manganese, desirable results are obtained in the use of large proportions of i manganese for the production of stainless steels,

very'little, if any, carbon being required to keep the alloy in the austenitic state. I, therefore,

accomplish the objects of my invention primarily 'by the use of a high purity manganese so that the final alloy will have a minimum carbon content and contain a minimum amount of impurities as silicon and aluminum, all as will be more fully described hereinafter.

For the convenience of those skilled in the art,

I shall refer more in particular to the character of the manganese employed.

For the purpose of comparison, I give below the analyses of two samples of commercial manganese:

20 austenitic alloy.

to retain the desirable austenitic state analyzed 0.54 to 0.79% silicon. Unquestionably the presence of this relatively large proportion of silicon, introduced probably through the use of a commercial source of manganese, accounted in part for the fact that 0.3% of carbon was required in this particular alloy. 4 A small proportion of carbon may be presen in the alloys of my invention without deleteri- 1 ously afiecting them. Indeed, up to 0.15% carbon may under certain circumstances be advantageous. At no time, however, do I havepresent more than 0.15% of carbon in the alloy as a whole.

The silicon and aluminum must be very carefully controlled in order to-produce satisfactory results with my invention because these two sub-n stances, normally found in commercial manganese, tend to prevent the production of an While it is possible that some silicon and aluminum may be introduced through the iron or chromium, in general, the production of these two materialshas been so controlled in the past that substantially silicon and aluminum free materials may be obtained. However, in so far as there may be silicon or aluminum present,

the proportion of these two impurities so found must be taken into consideration. In general, however, if the manganese is highly purified so as to contain less than .3% of the compounds iron, carbon, silicon and aluminum, very satisfactory results can be obtained by the practice of my invention. In general, the total of aluminum and silicon should not be more than 5 0.15% based upon the weight of the manganese present, and dependable results can be obtained if this figure be not exceeded.

While I have not been able to determine all of .the properties of such steels as fall within 40 the scope of my invention, I have found that the so-called epsilon phase very often found in high manganese content alloys is avoided and also conditions do not seem to favor the stability of the B constituent referred to hereinabove. Furcent 'thermore, apparently because of the absence of ,By purifying commercial manganese by either of the methods set out hereinabove. I have been able to produce a manganese product containing less than 0.1% of any of the elements iron, carbon, silicon and aluminum and wherein the sum of these elements present in the manganese is less than 0.3%. A manganese of this type, I have found, possesses entirely different properties in stainless steels than the manganese heretofore utilized in metallurgical work and presumably considered adequately pure for alloying purposes. The impurities present in the manganese, particularly silicon. and aluminum, if present in appreciable quantities, seem to be exceedingly powerful in preventing retention of the austenite.

It is significant that the stainless steel described in the article in The Metallurgist referred to impurities, other disadvantages, heretofore found when the manganese was increased app fgciably, do not seem to occur. When very high manganese is employed I have not determined fully whether or not the resulting alloy is entirely austenitic under all conditions of time and temperature employed in treatment, but it appears that the alloys are either substantially entirely austenitic or contain such appreciable quantities of the austenitic phase that they have excellent working properties and from a tensile strength and hardness standpoint they are suitable for many uses where stainless. steel is customarily used.

In a true type of stainless iron the nickel may,

as suggested, be replaced entirely by the manganese with good results, the proportion of manganese being increased with increase of chromium and reduction of nickel so that in the higher chromium substantially nickel free alloys, more than 30% manganese may be required to secure the desired properties described. An example of such an alloy is one containing 31 to 32% manganese, about 17 to 18% chromium, and

v balance iron. 7

While the stainless steels of my invention do not require the presence of copper or other materials heretofore employed in certain modified hereinabove and which requires 0.3% of carbon stainless steels, I may, if desired, add minor pm.

portions of other constituents such as copper, while still maintaining the balance of silicon, aluminum and carbon described.

The present application is a continuation-inpart of my prior application, Serial No. 199,602, filed April 2, 1938, now Patent No."2,216,535, issued October 1, 1940;

What I claim as new and desire to protect by Letters Patent 01 the United States is:

An austenitic stainless steel alloy of iron, chromium and manganese, the alloy as a whole having. less than 0,15% carbon, and not more than 0.15% total 01' silicon and aluminum based. on the total amount of manganese present, said alloy consisting of about 31% to 32% manganese,

10 minum.

JAMES a. LONG.