US2216535A - Stainless steel - Google Patents

Description

Patented'oct. 1, 1940 PATENT orrics s'ranvwss swam.

James B. Long, M1, assignor to hi cago Development mm. m. 111-, a corporation of Illinois I 1% Drawing. Application April 2, 1938,

Serial No. 199,602

action 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 efl'ective in changing the body centered ferrite of chro mium 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 18-8 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 extra- 40 polation 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% 4 of chromium 0.3% of carbon must be employed to give the desirable austenitic character to the alloy. P

The increase of the carbon content in alloys of .the type discussed has certain disadvantages.

50 In such alloys as 18% chromium and 10% manganese with suihcient carbon to insure completely austenitic properties, a hard brittle constituent is precipitated which has been tentatively identified as the B constituent of Bain and 55 Griflith. This compound is an iron-chromium compound, the stability of which is definitely questioned in pure alloys, Heycock reported an inability to find this compound in pure alloys. The useof increased carbon is also objectionable because increased carbon tends to cause susceptibility to inter-granular corrosion. Inhibi- 5 tors such as coiumbium 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 manga-- nese 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 thisdir'ection, 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 20 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 beendefinitely determined. Nevertheless, the very distinct presence of the two phases under conditions found in the past has necessarily resulted in in-' vestigators 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. I

One object of my invention is the provision of an improved austenitic stainless steel having as a constituent thereof a substantial quantity of manganese.

Another object is the utilization of manganese and nickel in such a manner as to assure ans-- tenitic properties with either no carbon or such 2 I a 2,210,5ss

a small amount of, carbon as to have little or no 5 nese in stainless steels.

In the development ofmy invention Lfound that so-called pure manganese as it was delivered commercially to experimenters in general contained as much as 6% of impurities, gen- 10 erally 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 and %%'of the manganese, I considered possibly to be of significant interest. I proceeded on the assumpticn that the silicon and aluminum content of manganese had to a considerable extent been overlooked by investigators and that this might to account to a substantial extent for the results 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 0 use of large proportions of 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 obiects of my-invention primarily by the use of a 3 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. '40 For the convenience of those skilled in the art I shall refer more in particular to the character of the manganese employed. V

For the purpose of comparison, I give belowthe analyses of two samples of commercial manll ganese:

By purifying commercial by either n of the methods set out hereinabove, I have been able to produce a u product containingJess than 0.1% of any-of the elements iron. carbon, silicon and aluminum and wherein the sum of these elements present in the manganese a islessthan 0.3% Amofthistypql have found. entirely different properties in stainless than the n heretofore utilised in metallurgical work and presumably considered adequately pure for alloying purn poses. The impurities presentinthem,

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 1; in the article in The llietallurgist" referred to hereinabove and which requires'0.3% of carbon 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 com- 5 h I mercial source of manganese, accounted in part for the fact that 0.3% of carbonwas required this particular alloy.

I found that the following range of proportions may be employed to produce substantially m the properties of 18-8 stainless steel, modified slightly, to be sure, when the extreme of the range indicated is employed: nickel 0 to 5%, manganese 10 to 30% or more, chromium 10 to and the remainder iron. The manganese which I employ in all instances contains less than 0.3% iron, carbon, aluminum and silicon and less than 0.1% of any one of these substances. h

A small proportion of carbon may be present 99 in the alloys of my invention without deleterie ously affecting them. Indeed, up'to 0.15% carbon may under certain circumstances be advantageous. At no time, however, do I have present more than 0.15% of carbon in the alloy as 35 h .a whole.

The silicon and aluminum must be very carefully controlled in order to produce satisfactory results-with my invention because these two substances, normally found in commercial mangaa. nese, tend to prevent the production of an austen'itic alloy. While it is possible that some silicon and aluminum may be introduced through the iron or chromium, in general, the production 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 thesetwo impurities so found must be taken into consideration. In general, 4 however, if the manganese is highly purified so as Y to contain leasthan 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 0.15% based upon the weight of the manganese present, and dependableresultscanbeobtainedifthisflgure benot exceeded.

The stainless steel produced in accordance I my invention is obtained entirely in the austenitic stage by ordinary cooling. This stainless steel or stainless iron canbeproducedby.in general,

substitutingforallorapartofthenickelheretofore employedinstainlesssteelanequalor greater proportion of manganese purified to such a state that carbon, silicon and aluminum present in the manganese in less than 0.3%. It isnotto be assumed that my invention islimitedtothepresenceoflS'lGof ssreferredtointhespecificexampiegivenhereinabove. I may employ or more of substan-- tially chemically pure such as that described and produce steels havingv excellent properties. WhileIhavenotbeenabletodetera mine all of the properties of such steels, I'have found thatthe 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 1B constituent referred to here- 7. inabove. Furthermore, apparently because of the absence of. impurities, other disadvantages heretofore foimd when the manganese was increased appreciably, 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.

18%, nickel 2% and balance iron is entirely austenitic and has very excellent properties making it very suitable for use in the industries where high corrosion resistance and strength are desired. 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.

While the stainless steels of my invention do not require the presence of copper or other materials heretofore employed in certain modified stainless steels, I may, if desired, add minor proportions of other constituents such as copper while still maintaining the balance of silicon, aluminum and carbon described.

WhatI claim as new and desire to protect by Letters Patent of the United States is:

1. An austeniticstainless steel alloy of iron, chromium, nickel and manganese, said alloy as a whole having less than 0.15% carbon and less that 0.15% total of silicon ,and aluminum based upon the manganesecontent, said alloy consisting of manganese, to 30%.; substantially pure chromium, 10 to 20%; nickel, .5% to 5%; balance, substantially pure iron; said manganese being-of a highly pure grade and containing not more than about .1% by weight of any one of the elements carbon, silicon and aluminum.

2. An austenitic' stainless steel comprising nickel up to about 5%, manganese from 10 to chromium from 16 to 20% and the re- .mainder iron, said alloy as a whole having less than 0.15% carbon, 'and the total content of sili- 'con and aluminum being less than 0.15% based upon the manganese present, each of said ele-