US1914083A - Nitrided spring - Google Patents

Description

.lune 13, 1933. y G M- EATON 1,914,083

NITRIDED SPRING Filed June 8, 1952 PEQJEC rfa Wfw dr' INVENTOR Patented June 13, 1933 UNITED STATES PATENT OFFICE GEORGE IMI. EATON, F BEN AVON, PENNSYLVANIA NITRIDED SPRING Application filed .Tun-e 8, 1932. Serial No. 616,048.

ated in the critical region of the coil when subjected to stress during coiling, and dur. ing the thermal treatments employed as well as in the normal Working of the spring.

The surface defects are of a nature peculiarly susceptible to magnification durin the rapid heat cycle attendant upon a quenc ing operation and the primary object of this invention is to prevent or substantially reduce the exaggeration of the surface defects in helical springs.

I have found that by relieving the steel of the shock from quenching, the conditions which are mainly responsible for the epidemic` failures of helical springs are substantially removed.

The type of failures herein referred to and the conditions that cause them may be better understood in connection with the accompanying drawing constituting a part hereof, in which like reference characters designate like parts, and in which:

Figure 1 is a side elevational viewpartially in section of a typical lcoil spring showing the type of failureto be eliminated or delayed; 1

Figure 2 is a view diagrammatically illustrating a stress concentration at the inside of coil; 21nd Figure 3 illustrates cross-sectional views showing ideal structure distribution when produced without local stress in accordance with this invention and structure distribution obtained by conventional heat treatment.

I employ a steel, the analysis of which lends it to nitriding to protect the surface of the 0 spring against atmospheric corrosion and which at the same time is adaptable to air hardening` to a degree which gives adequate characteristics .to the material in the core of the spring rod or wire.

I have discovered that alloy steels of the following analysis are suitable for this purpose:

Carbon 0.15 to 0.35 Manganese '4 0.40 to 0.80 60 Aluminum 1.00 to 2.50 Molybdenum 0.40 to 1.00 Sulphur Low. Phosphorus Low Silicon Normal The steel rod is centerless ground, then coiled to the size and shape of the helical spring desired, as shown in Figure l, and is then subjected to a normalizing treatment in a non-decarburizing atmosphere to relieve the coiling stresses if essential. The centerless grinding of the spring rod removes or reveals the surface seams or streaks designated by the parallel arrows in Figure 1 of the drawing and prepares the surface to receive the nitrided case. The coil is then nitrided by heating it in an atmosphere of ammonia gas at a temperature of about 10000 F. for a suicient period of time to produce a case of suitable depth. Thermally the nitriding treatment constitutes a drawback from the normalizing treatment and leaves the steel in a strong and tough condition quite ideal for the core of the spring rod. Thev slow rate of cooling involved in the nitriding process produces no transient surface tension with danger of incipient or actual cracking as in quenched springs.

In making the highest quality springs it may be desirable vto subject the material to a thermal pre-treatment for structural refinement, and the spring stock should have its decarbonized surface removed as by cenj terless grinding before subjecting it to the nitriding process. Structural refinement is for the double purpose of adapting the steel to take the nitriding treatment, and to resist service stresses. Centerless grinding may be omitted for lower grade springs if the laylo er of decarbonized material is of immaterial thickness, but any material decarburized layer will damage the character oi? the nitrided case.

The nitrided spring possesses an outer shell of extreme hardness and uniform depth as shown in the section marked (a) in Figure 3 of the drawing which has such toughness that the shell will resist cracking even when the entire body is subjected to some degree of permanent distortion and which possesses an elastic limit far above that of any other steel which has been heat-treated at a temperature not exceeding 1000o F. Such springs be* cause of their smooth hard shell free from incipient surface flaws or cracks can be Worked close to the elastic limit of the material in shear since the shearing stress Will not be concentrated on a Weak surface portion to cause failure.

The spring can be accurately formed prior to nitriding, and in precision springs when they are of correct shape and free from internal stress, the spring after hardening Will be of substantially the identical shape as the unhardened spring.

The sections (b) and (c) of Figure 3 show structure distribution which are the best that heat treatment can produce as compared with the structure distribution shown by section (a) which is produced by the process herein described.

Although l have specified molybdenum alloys as particularly adapted for helical springs it will be obvious that other steels suitable for nitriding, may be employed Within the spirit and scope of my invention.

l claim:

1. A nitrided steel helical spring containing by analysis:

Carbon 0.15 to 0.35 Manganese 0.40 to 0.80 Aluminum 1.00 to 2.50 Molybdenum 0.40 to 1.00

2. As an article of manufacture a nitrided steel helical spring.

ln testimony whereof I have hereunto set my hand.

GEORGE M. EATON.

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