US2531720A - Chromium stainless steels - Google Patents

Description

Patented Nov. 28, 1950 CHROMIUM STAINLESS STEELS Mary Baeyertz, Chica Lindsey,

go, 111., and Willard M. Rumford, R. I., assignors to Carnegie- Illinois Steel Corporation, a corporation of New Jersey N Drawing. Application June 21, 1946, Serial No. 678,361

1 Claim. 1

This invention relates to chromium stainless steels and, particularly, to chromium stainless steels which are subjected to repeated heating and pickling when being formed to make certain shapes or articles possess exceptional stability to localized corrosive attack induced by a combination of heating followed by application of corroding media.

Steels of the aforementioned class containing, substantially, 5 to 14 per cent chromium, and 0.05 to 0.25 per cent carbon, are widely used in industry for manufacturing articles intended to possess reasonably high mechanical properties and good creep-resistant characteristics associated with a marked corrosion resistance. Industrial considerations advance forging as the preferred method of shaping thereof, and hot forming under a hammer or in dies occupies at present the foremost position among methods used therefor.

Frequent intricate configuration of such forgings often calls for several forging operations. Since closeness of forging dimensions is usually imperative, the industry adopted the practice of removing scale formed on forgings prior to sub- ,iecting them to a subsequent forging step. Pickling in inorganic acids or solutions thereof may be held as the preferred method for achieving aforementioned scale removal, and is widely used in forging practice.

Extensive industrial experience demonstrated that steels of the class described do not respond to said pickling in an identical manner. Some of said steels soaked for one hour at 1950 F., forged, air cooled and pickled in a 50 per cent H01 solution at 170 F. until the scale is removed, develop uniform clean surface. Identical sequence of the same steps produces on the surface of other steels, still fully within the compositional range of aforementioned alloys, a plurality of disoriented surface discontinuities having macroscopic dimensions and a shape which justified attributing thereto the term wrigglers in the plant parlance. The surface appearance thereby produced closely resembles the surface of burnt steel forgings fully familiar to those skil ed in the art.

We have found the explanation thereof in a critical relation between chromium and carbon content. The ranges involved therein are quite narrow and, when the steel must meet certain mechanical-properties specifications while remaining free from wrigglers, may render steel manufacture extremely diflicult.

For the purpose of illustration, there may be used a case where desired properties require the composition of 0.0? to 0.12 per cent carbon, 11.50 to 12.50 per cent chromium and 0.40 to 0.60 per cent molybdenum, other elements being present in the amounts common to the grade, and freedom from aforementioned surface disconinuities is specified. Our investigations demonstrated that utilization of the whole range of carbon and chromium inevitably leads to the failure of meeting the requirements with regard to immunity to the surface discontinuities known as wrigglers. These are prone to occur when the carbon falls within the upper limits of the range or the chromium falls in the lower part of specifled range.

The requirements can be met in the above case only when the chromium range is restricted to 12.0 to 12.5 per cent, and carbon is limited to 0.07 to 0.09 per cent. It is easily seen that working a heat within three points of carbon commands an exceptionally close control of operating steps, and the diiiiculties involved therein increase the percentage of rejected heats to the point prohibitive of commercial operation.

We have found that the ratio between the permissible carbon content and the percentage of chromium inducive to the desired freedom from surface discontinuities remains substantially the same with the changing percentage of chromium. Larger amounts of the latter element present in steel permit the use of a higher carbon content. However, the narrow range of substantially three points of carbon for the variations of chromium encompassed by one-half of one per cent thereof remains a prerequisite for an adequate performance of the steel, with all inconveniences inherent thereto.

We have found that the necessity for such exceptionally close control of carbon is eliminated by addition to the steel of a certain critical amount of elements belonging to the group: titanium, columbium, and boron, without any detrimental eiiect on the physical or corrosionresistant characteristics of the metal.

Permissible and desirable range of concentrations of the aforementioned elements is specified by two limits, namely, the presence of a sufiicient minimum for inhibition of surface discontinuities development, and the avoidance of the percentage thereof inducive to change in the response of steel to normal heat treating operations. Thus such elements should not be present in such concentration that a substantial amount of austenite is retained on COOIiIlg after forging.

The lower concentration limit of said additional elements depends on both carbon and chromium content of steel. For steels containing, for ex ample, 11.0 to 12.5 per cent chromium and 0.08 to 0.11 per cent carbon, a minimum columbium content of substantially 0.25 per cent, titanium of substantially 0.08 per cent, and boron of 0.003 per cent have been found adequate for the elimination of surface discontinuities after repeated heating and pickling, at least after seven repetitions of the cycle. 7

Our studies of the properties of steel having the aforementioned exemplary composition as a function of increased percentages of titanium, columbium, and boron alloyed with said steel, indicated that the properties undesirable from the standpoint of the application of said steel to industrial uses begin to develop when columbium content exceeds substantially 1.00 per cent, titanium percentage reaches substantially 0.50 per cent and boron concentration becomes greater than about .05 per cent.

Steels embraced within the scope of the present invention include alloys generally known as chromium stainless steels and which comprise, besides the principal constituents, namely iron, chromium, and carbon, a certain percentage of manganese, silicon, molybdenum, nickel and other elements present in the concentrations insuflicient, in the combination thereof, for transforming said steels to stable austenitic alloys.

Other modifications of applying the principles of our invention may be employed within the scope of the appended claim.

We claim:

The method of producing forged articles posed. .of 11 to 12.5% chromium steel free from surface discontinuities comprising adding between .003 and 05% boron to straight chromium steel containing between 11 and 12.5% chromium and 0.8 to 11% carbon, said boron addition being 4 regulated to prevent the formation of surface discontinuities on said steel resulting from acid pickling to remove scale resulting from heating to forging temperatures, forging articles from blanks of said steel, said forging comprising repeatedly heating said blanks to forging temperature, forging at said temperature, cooling and acid pickling the same intermediate each heating to remove the scale formed by heating to forging temperature.

MARY BAEYER'IZ.

WILLARD M. LINDSEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,519,388 Walter Dec. 16, 1924 2,110,891 Reitz et al Mar. 15, 1933 PATENTS Number Country Date 375,793 Great Britain June 20, 1932 OTHER REFERENCES The Book of Stainless Steels, 2nd Edition, Edited'by Ernest Thum; pages 264 and 268. Published in 1935 by the American Society for Metals, Cleveland, Ohio.

Stainless Iron and Steel, pages 59 and 852. Edited by Monypenny. Published in 1931 by Chapman-Hall, London, England.

Advantages of Columbium in Wrought 4 to 6 Chromium Steel, pages 10 and 16. Published in 1937 by the Electro-Metallurgical (30., Niagara Falls, New York.