US2083208A - Structural parts having high permanent strength - Google Patents

Description

Patented June 8, 1937 rSA-TENT creme I 2,083,208 STRUCTURAL mars nsvmeflqnlon PER;

MANENT STRENGTH Walter Tofaute, Essen, Germany, assignor to The Nitralloy Corporation, a corporation of Delaware N0 Drawing. Application September 17, 1934,.

Serial No. 744,365. in Germany September 18,

3 Claims.

This invention relates to structural parts, such as screws, bolts and tubes adapted for use in power plant installations and similar situations where high resistance to pulling or tension stresses at elevated temperatures is a desideratum.

It has been found that the strength of iron and steel alloys at elevated temperatures and their resistance to creep at such temperatures can be greatly increased by subjecting them to an appropriate nitriding treatment. This resistance to stress and to creep at elevated temperatures is referred to hereinafter as permanent strength and is to be understood as the load in kg./m1n.

which an iron or steel alloy will support at a temperature of 500 C. without showing an extensibility or rate of creep of more than 0.0005% per hour in the 25 to 35 hour period of the tension test.

In the conduct of the nitriding treatment for the purpose of imparting high permanent strength, it is not important that a high surface hardness be produced. The purposes of the present invention are accomplished by so conducting the nitriding operation, or so choosing the iron or steel alloy that is to be treated, that the depth of the nitride case is relatively high as compared with the present nitriding practice. It has been found that many iron and steel alloys heretofore regarded as unsuitable for nitriding because the obtainable surface hardness is relatively low, may be caused to acquire a high permanent strength when nitrided. In other words, although the surface hardness imparted by nitriding is relatively low the nitride depth that can be imparted is such as to insure a satisfactory permanent strength and resistance to creep at high temperatures. For example, many aluminum-free chromium steels show a relatively low hardness when nitrided, but nevertheless acquire a nitride case of relatively great depth and consequent high permanent strength. As an example, a steel alloy of the following analysis has been tested:

A test piece of this steel with a diameter of 18 mm. was cooled from 900 C. in air, drawn to 700 C. and then nitrided at 500 C. for hours by treatment with ammonia gas. This test piece showed a nitride depth of 1.1 mm. and a permanent strength of 21.8 kg./mm. A test piece of the above steel alloy heat treated in like manner but not nitrided showed a permanent strength of 13.8 kg./mm'.

Steel alloys which when nitrided at a normal nitriding temperature, say at about 500 C., form a very hard but comparatively thin nitride layer, as for example, the chromium-aluminum steel alloys, undergo when nitrided at such a temperature little or no substantial improvement of the permanent strength. It has been found that when these alloys are nitrided under conditions such as to produce a relatively great nitride depth and at the same time decreased hardness (for example, of about 600-800 according to the Firth hardometer), a highly satisfactory increase in permanent strength is obtained. This result may be obtained by carrying on the nitriding step at a relatively high temperature, for example, at

- about 550 to- 600 C., or, if desired, at an even higher temperature up to about 800 C. In order to illustrate this, there are recited below the results of tests on steel alloys of the following compositions:

C Si Mn Or Al Mo Steel I 0. 33 0. 31 0. 76 l. 00 1. 08 0. 25 Steel II 0. 29 0. 16 0. 38 l. 39 1. 18

C. in oil and drawing to 650 C. showed a permanent strength of 4 kg./mm. Another test piece after a similar heat treatment followed 'by nitriding at 560 C. for 100 hours showed a permanent strength of 10.4 kg./mm. with a nitride depth of 0.97 mm.

For producing structural parts according to the present invention, iron and steel alloys that are especially adapted for the purpose are those which have a carbon content up to 1.5% and contain up to about 3% silicon, up to 3% manganese, up to 20% chromium, up to 3% aluminum, up to 3% molybdenum, tungsten, titanium, vanadium or zirconium, and. up to 20% nickel or cobalt. The hardness of the nitrided alloys in the following claims is expressed in terms of measurements made with a Firth hardorneter.

I claim:

1. A structural part subject in use to prolonged tension stresses at elevated temperatures, formed of a ferrous-base alloy having a nitrided case having a hardness between 600 and 800 penetrating to a depth of about 1 mm. or more, and characterized by a permanent strength in excess of an identical unnitrided part formed of the same alloy.

2. A structural part subject to prolonged tension stresses and adapted for use at elevated temperatures, formed of an aluminum-free chromium-containing steel alloy having a nitrided case having a hardness between 600:-a nd 800,

penetrating to a depth of about 1 mm. or more, and characterized by a permanent strength in excess of an identical unnitrided part formed of the same alloy.

3. A structural part subject to prolonged tension stresses and adapted for use at elevated temperatures, formed of a ferrous-base alloy containing chromium and aluminum in effective proportions but not exceeding about 20% of chromium and about 3% of aluminum, said part having a nitrided case having a hardness between 600 and 800 penetrating to a depth of about 1 mm. or more, and characterized by a permanent strength in excess of an identical unnitrided part formed of the same alloy.

WALTER TOFAUTE.