US2134670A - Corrosion resisting ferrous alloys - Google Patents

Description

Patented Oct. 25, 1938 coanosron aasrsrme rsnaous anno s James A. Parsons, Jr., Dayton, Ohio, asslgnor to The Duriron Company, 1110., Dayton, Ohio, a corporation of New York No Drawing. Original application December 28,

1935, Serial No. 56,586. Divided and this application September 20, 1938, Serial No. 230,889

6 Claims.

This invention relates to corrosion-resisting ferrous alloys; and it relates more particularly to ferrous alloys or alloy steels characterized by a relatively high content of nickel and chromium,

together with a further content of supplementing or fortifying constituents comprising copper, molybdenum and silicon in carefully adjusted but somewhat variable percentages that are comparatively small, molybdenum being always present in substantially greater proportion than silicon. The new alloys are characterized by a high degree of resistance to corrosion and, in addition, by the fact that sound castings can be produced therefrom and that they may also be forged and rolled hot with far less loss than has been unavoidable with alloy steels of generally comparable type heretofore known.

The present application is a division of prior I copending application, Serial No. 56,586, filed December 28, 1935.

Corrosion-resistant austenitic alloy steels of high nickel-chromium content, fortified or reinforced with small amounts of silicon, copper and molybdenum, have been heretofore known and used for certain purposes. But the range of theirpractical utility has been greatly restricted because they possessed certain inherent troublesome characteristics rendering them unsuitable for certain purposes or their fabrication into required products very expensive. has been very diflicult to produce from such alloys satisfactory sound castings, such as valve and pump parts, that must be tight against hydrostatic pressure. Furthermore, in hot forging and rolling ingots of such prior alloy steels into bars, rods and sheets, for instance, the yields obtainable in practice seldom exceeded 50 per cent and were commonly lower, due to excessive but unavoidable waste, thus greatly increasing the cost of the forged or rolled products and naturally limiting their use to special situations where such high cost would not be prohibitive.

It has now been found that the foregoing objections and disadvantages characterizing such prior alloy steels can be largely overcome by properly adjusting and controlling the relative proportions and total amounts of the before-mentioned reinforcing or fortifying constituents, that is, silicon, copper and molybdenum, in the manner hereinafter set forth; and that when this is done, the mechanical and physical characteristics of the resultant alloy steels are greatly improved. Furthermore, their corrosion-resistance is not only fully as good as that characterizing prior alloy steels of this general type but, in the best For example, it

embodiment of the invention, is substantially enhanced.

The percentages of nickel and chromium characterizing ferrous alloys of the present invention, although capable of fairly wide variation, are relativelyv large in all cases. The novel alloys with which the present application is particularly concerned and which have especially outstanding advantages and utility in the practical art, never contain substantially less than 20 per cent nickel and 17 per cent chromium as mlnima. The nickel content may run as high as 23 per cent and the chromium content as high as 22 per cent, nickel always exceeding chromium in the alloy composition. Best results are obtained when the proportion of nickel is between 21 and 22 per cent, and that of chromium is between 18 and 21 per cent. Iron, the largest single component, always constitutes at least about per cent of the alloy and usually more than per cent.

Although the marked effect of copper in high nickel-chromium steels as a fortifying agent to increase corrosion resistance has long been. recognized, its employment for this purpose, even in very small proportions, has been attended heretofore with serious difficulties and objections, especially because of its adverse effect upon the hot-working properties of such alloy steels through a tendency to produce hot shortness.

According to the present invention, the fortifying or reinforcing elements are so proportioned that the highly desirable effect of copper in increasing corrosion-resistance can be utilized fully while at the same time eliminating or sufficiently reducing its tendency to produce hot shortness. It has been found that, in order to accomplish this, it is vital to have the percentage of molybdenum always substantially greater than that of silicon, and most desirably at least two or more times as great. The percentage of silicon should not be substantially less than 0.50, and as much as 2 per cent is permissible in practice. It is generally very much better, however, to keep the silicon well below 2 per cent, and indeed not to let it greatly exceed 1 per cent. For best results, the molybdenum content of the new alloy steels should be in the neighborhood of 3 to 3.5 per cent, ranging normally in practice from 2.0 to 4.0 per cent. It is very important that the total or combined percentage of molybdenum and silicon content shall not substantially exceed 5 per cent, an upper limit of 4.5 per cent being ordinarily distinctly preferable to observe. A higher aggregate percentage of these constituents is found to have a pronounced unfavorable effect upon the hot working properties of the alloy.

Alloy steels of the present invention are also characterized essentially by low carbon content not substantially exceeding 0.12 per cent. Indeed, it is only in exceptional cases that it is advisable to permit the carbon content to exceed 0.10 per cent. Ordinarily it is found that about 0.06 to 0.08 per cent carbon is extremely satisfactory in most cases, and a range of 0.04 to 0.07 per cent may be regarded as optimum. Too high a carbon content is to be avoided because it increases vulnerability of the alloy to intercrystalline corrosion, especially upon exposure to contact with sulphuric acid.

It will be understood that the new ferrous alloys may contain manganese in variable percentages, e. g. 0.40 to 0.75 per cent; also minute quantities of the common impurities such as phos phorus and sulphur which, together, do not amount to more than 0.04 per cent or thereabouts in the type of iron suitable to use in manufacturing alloy steels of the type here in question.

By maintaining the ratio of molybdenum to silicon such that the molybdenum always substantially predominates, with their combined percentages not substantially exceeding 4.5 per cent, and by observing the other precautions herein set forth, it becomes possible to employ, for example, from 1 to 3 per cent of copper in an alloy steel containing, say, 18 to 20 per cent chromium and from 22 to 23 per cent nickel, which is of particularly great industrial value not only because of its high degree of corrosion resistance to sulphuric acid and other drastic corroding agents over a wide range of practical conditions, but also because sound, tight castings can be made from, it and, furthermore, it can be easily forged and rolled with remarkably high yields of desired products. As a rule, such an alloy steel contains at least 50 per cent iron.

Heretofore, in order to produce an alloy steelof this general type giving a for ng y eld of even 50 per cent, it was necessary to keep the copper content down to about 0.3 per cent as a maximum. The ability to use a much higher copper content in such alloy steels and thus to enhance materially their corrosion resistance, while obtaining not only as good but far higher forging" and rolling yields, is of the greatest importance industrially.

In order to illustrate further the underlying principles of the invention, and without intending thereby to limit its scope, the compositions or analyses of a number of the new alloy steels that are typical and representative are given in the following table showing percentages of the essential components other than iron:

Example No. Ni Cr 8i Cu Mo 2). 0 19. 0 0. 1. 0 3. 5 0.07 (QM-0m). 22 0 21. 0 0. 5 0. 5 3. 5 20. 0 18. 0 l. 0 1. 0 3. 5

21. 0 18. 0 1. 0 1. 0 3. 0 22. 0 21. 0 1. 0 0. 5 3. 5 Z). 0 17. 0 1. 5 l. 0 3. 5 20. 0 l7. 0 l. 5 0. 5 3. 5 R 0 22. 0 1. 0 1. 0 2. 0 Z). 0 18. 0 0. 5 1. 0 4. 0 21.6 20.3 0.5 0.9 3.4 0.07 21. 0 20. 8 0. 5 1. 0 3. 5 0.12 at 8 l8. 5- 1. 0 O. 0 3. 2 0.06 21. 5 17. 8 1. 5 1. 0 3. 1 0.07 21. 5 17. 5 l. 4 0. 9 3. 0 01B 21. 1 17. 4 l. 5 0. 9 3. 2 0.00 21. 6 18. l 1. 0 1. 0 2. 0 0.06 21. 5 l8. 1 l. 3 2. 8 3. 4 0.07

Alloy steels of the character represented by the foregoing analyses are all characterized by the fact that the reinforcing elements relied upon to give increased corrosion resistance as compared to ordinary high chromium-nickel steels are carried in a uniform, stable, solid solution even at ordinary temperatures. When subjected to drastic tests with the usual corroding reagents under conditions ranging from reducing through neutral to strongly oxidizing, and at widely varying temperatures, the new alloy steels show extremely high corrosion resistance, notably so toward sulphuric acid. Their mechanical properties are also excellent. Whereas standard bend test specimens of prior alloy steels of generally comparable type fail at less than 90 when bent cold, similar test specimens of the new alloy steels show no cracking or other failure when bent 90 nor, in many cases, even when bent 180. This is evidently due to the superior microstructure of the new alloy steels which is found upon examination to be homogeneous in character, there being little or no precipitation of ferrite on the crystallographic planes or intracrystalline faces. Also there is practically no formation of insoluble carbides at the grain boundaries; and accordingly these new alloy steels are especially free from attack through intergranular corrosion. Thin sections of castings produced from the new alloys are much less or not at all subject to the severe coring or double wall effect, commonly characterizing castings from comparable prior alloy steels, which coring is not .only conducive to porosity (leaky castings) but lowers corrosion resistance as well as the elastic limit of the metal.

Alloy steels having analyses approximately as given in the table given hereinabove are regarded as particularly advantageous embodiments of the invention because combining very high corrosion resistance with exceptionally good casting and hot working characteristics. For example, in actual mill operation with an alloy steel having substantially the analysis given in Example No. 10, the yield from poured ingot to finished rolled bar was 73 per cent in a typical instance. contrasts with a maximum obtainable yield of only about 50 per cent when rolling prior alloy steels of most nearly comparable type. Similar marked increases in forging and rolling yields are characteristic of the new alloy steels generally.

The new alloy steels may be manufactured by ordinary methods well known to those skilled in the art of making high nickel-chromium alloy steels, no special expedlents or precautions being required. 80 also, production of castings, ingots, bars, sheets, etc., from these steels, and the heat treatment thereof, may be carried out in the usual way. It it accordingly unnecessary to describe these methods here.

The term wrought as employed in certain of This the claims hereinafter is' to be understood as in its usual generic sense to include working metal as by forging or rolling, for example.

What is claimed is:

1. An alloy comprising the following elements in proportions falling within ranges of percentages substantially as follows:

substantially all the balance to make 100 per cent being iron, which is never less than per cent, molybdenum always being in excess of silicon and the sum of molybdenum and silicon never exceeding 5 per cent, nickel always being in excess of chromium, said alloy being corrosion resistant and possessing good hot working properties, a standard test specimen of said alloy being capable of withstanding a 90 cold bend without failure.

2. An alloy comprising iron, at least per cent; chromium 18 to 21 per cent; nickel 21 to 22 per cent and predominating over the chromium; carbon, not exceeding 0.10 per cent; copper, 0.75 to 1.25 per cent; molybdenum, 3 to 3.5 per cent; and silicon 0.5 to 1.5 per cent, said alloy being corrosion resistant and possessing relatively good hot working properties, a standard test specimen of said alloy being capable of withstanding a cold bend without failure.

3. An alloy steel having approximately the following percentage composition: nickel 21.6, chromium 20.3, silicon 0.5, copper 0.9, molybdenum 3.4, and carbon 0.07, substantially all the remainder being iron, said alloy being corrosion resistant and possessing relatively good hot working properties, a standard test specimen of said alloy being capable of withstanding a 90 cold bend without failure.

4. An alloy steel having approximately the following percentage composition: nickel 21, chromium 18, copper 1, silicon 1, molybdenum 3, and carbon 0.07; practically all the remainder being iron, said alloy being corrosion resistant and .possessing relatively good hot working properties, a

35 standard test specimen of said alloy being capable of withstanding a 90' cold bend without failure.

5. A corrosion resistant wrought ferrous metal article having the following elements in proportions falling within ranges of percentages substantially as follows:

Per cent Nickel 20.00 to 23.00 Chromium 17.00 to 22.00 Silicon 0.50 to 1.60 Copper 0.50 to 2.80 Moly n 2.00 to 4.00 Carbon up to 0.12

Per cent Nickel 20.8 Chromium 18.5 Silicon- 1.0 Copper 0.9 Molybdenum 3.2 Carbon 0.08

the balance to make per cent being substantially all iron.

' JAMES A. PARSONS, Jn.