US2471080A - Austenitic steel having high hot hardness - Google Patents

Description

Patented May 24, 1949 UNITED STATES TENT OFFIQE AUSTENITIC STEEL HAVING HIGH HOT HARDNESS Carl B. Post, Wyomissing, Pa., assignor to The Carpenter Steel Company 1 Claim. 1

This invention relates to improved austenitic steel having high hot hardness and adapted for use in making motor valves and valve parts, particularly multiple piece motor valves and parts for internal combustion engines and to improved steels.

The exhaust valves in modern internal combustion motors designed for increased power and higher speeds must resist the combined action of heat corrosion, wear and stresses to a high degree. In general only valves (and especially valve heads in multiple piece valves) made from austeniti-c stainless steels are capable of operating as exhaust valves in the modern, higher power, high speed internal combustion engines. The austenitic steels are able to withstand severe valve stresses and the high temperature oxidation and corrosive effects of the exhaust gases better than the so-called martensitic types.

The damaging stresses imposed on a motor valve during operation are due to two main effects: 1) the large temperature gradients found between the valve head, seat and stem; and (2) the high valve spring pressures used to speed up the action of the valve to keep pace with the design of higher power, higher speed motors.

It is known to those skilled in the art that motor valves made from the austenitic types of stainless steels have less susceptibility to cracking or tearing than martensitic types. Nevertheless cracking is still a major problem in the manufacture of motor exhaust valves for longer continuous service.

In many cases such valve failures due to this cracking can be attributed to a corrosionfatigue phenomenon; i. e., a combined action of stresses and the oxidizing action of hot corrosive exhaust gases on the valve at the region of the seat and just below the seat toward the stem. Such cracking or tearing of the valve head are initiated at or slightly below the valve seat where the temperature and COIIOSiVe effects of the gases attack the valve at its most highly stressed region. In exhaust valves made from ordinary austenitic stainless steel containing .20% carbon, 1.00% manganese, 1.00% silicon, 21.00% chromium and 12.00% nickel, I have observed that a relatively large number of such corrosionfatigue failures start from or are initiated at a cluster or stringer of carbides on the valve surface, either on the seat or immediately below the seat. Such carbide clusters represent a zone of weakness on the surface of the valve especially for creep stresses. It also has been my observation that the hot gases of the leaded gasolines attack the carbides in preference to the chromenickel matrix.

Exhaust valves made in accordance with the 2 present invention from an austenitic stainless steel having the compositions described herein, with certain critical and restricted amounts of nitrogen uniformly distributed throughout the steel, show a decrease or scattering of the excess carbides and an increased resistance to the corrosion-f'atigue type of motor valve failure.

They" also show a unique increase in hardness at room and elevated temperatures, by which is meant a temperature range of about 1000 to 1500 F. The present invention makes possible a valve having a hardness of 35 E0. on the seat and stem and 40 Rc. on the tip end. Those values are obtained at room temperature and what is even more important high hardness values are obtained at the high temperatures at which the valves of present day internal combustion engines are operated. The lack of such high hardness both at room temperature and high temperatures has been a chief obstacle to the general adoption in the industry of valves made from austenitic stainless steels. The performance of some motors and the high valve spring pressures employed therein establishes a necessity for room temperature and hot hardness of a high order of magnitude and it has been necessary tov sacrifice such resistance to tearing and corrosioncracking as has been obtainable by the use of the conventional or known austenitic stainless steels, and to use the martensitic type of Valve steel. The present invention makes possible the production of valves which have a combination of two outstanding properties, (a) improved resistance to tearing and corrosion-cracking, i. e. improved in relation to those made from the conventional or known types of austenitic stainless steel, and (b) high hot hardness as well as high hardness at room temperature. The increase in room temperature hardness of unfinished valves made from the analyses hereinafter disclosed leads to better machinability as compared to those valves made from the regular .20% carbon, 21.00% chromium and 12.00% nickel austenitic stainless steel without nitrogen, which in turn leads to lower manufacturing costs. The increase in hot hardness enables the valves and parts to resist warping due to the high valve spring pressures employed in modern internal combustion motors. Moreover, the cost of manufacture of motor valves of my invention is reduced because the higher hardness as forged or annealed leads to a substantial reduction in the stringiness of the austenitic matrix and aids considerably in reducing machining and grinding costs.

The proportions of chromium, nickel, carbon and nitrogen in the austenitic stainless steel used for these improved high temperature motor exhaust valves are shown by the following example:

Iron Substantially the :balance Typical steels within the scope of this invention, exemplified for purposes of illustration by a steel identified as No. 7331 (containing carbon 0.18, manganese 1.33, silicon 0.90, phosphorus 0.009, sulfur 0.013, chromium 20.91, nickel 11.07

and nitrogen 0.18 all in per cent by weight and the balance substantially iron) have improved properties shown by the subjoined data in comparison with a conventional. 21-12 steel identified as No. 2&1 (containing carbon 0.18, manganese 1.30, silicon 1.04, phosphorus 0.015, sulfur 0.008, chromium 20.98, nickel 11.09, and the balance substantially iron).

Brinell hardness Cogged 1700 1900 2100" N0. 7241 229 212 187 170 No. 7331 302 229 229 196 Specimens were completely non-magnetic after all of the above treatments (i. e., austenitic) W. T. means water treated, i. e. water quenched.

Tensile strength at room temperature as water treated from 1900 F.

Yield Point, Pounds Per Square Inch 52, 500 75,000 Tensile Strength, Pounds Per Square Inch. 97, 500 123, 500 Per Cent Elongation in 2" 49 42 Per Cent Reduction of area 61 60 Brinell Hardness 187 241 .4 Note that the yield point and tensile strength have been greatly increased without affecting the per cent elongation and per cent reduction of area, i. e. markedly higher tensile strengths and yield points are obtained without material impairment of ductility.

[Hot Brinells at 1200" and 1400 F. Water treated from 1900 F., tested at 1200 and 1400 F.]

Brinell hardness ggfl 1200 1400 F.

The corrosion rates of the steels of this invention are also materially less than in the case of the conventional 21 chromium 12 nickel steel.

I claim:

An austenitic stainless steel having a high hot hardness exemplified by Brinell hardnesses of 131 at 1200 F. and 113 at 1400 F. respectively, said steel having the following composition:

Per Cent Carbon 0.15 to 0.25 Manganese 1.00 to 1.50 Silicon 0.50 to 1.00 Chromium 20.00 to 22.00 Nickel 10.00 to 12.00 Nitrogen 0.15 to 0.20 Phosphorus (maximum) 0.02 Sulfur (maximum) 0.02

Balance iron except for incidental impurities CARL B. POST.

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

UNITED STATES PATENTS Number Name Date 2,121,391 Arness June 21, 1938 2,223,659 Harden et a1 Dec. 3, 1940 2,438,824 Rich Mar. 30, 1948