Heat Treating, Setting And Shot-peening Calls For Mechanical Springs

Author:  Zimmerli, F.P.
Source:  Metal Progress, June 1952, V. 67, No. 6
Doc ID:  1952012
Year of Publication:  1952
Abstract:  
Whether or not a spring is formed in the hard or soft condition it is usual to heat treat it. For soft formed springs either martempering, austempering or the conventional quench and draw will bring the spring to the desired hardness. Austempering and martempering are used when warpage due to quenching may be severe. Austempered springs, however, do not have the stress range of springs hardened by the other methods. If the material is hard when formed, it is usual to stress relieve to remove residual forming stresses. This operation also develops a more positive elastic limit. When springs are held under stress conditions at temperatures above 200 Deg. F., an equilibrium of temperatur versus percent load loss is obtained. Each material has definite limits when equilibrium is lost and the set becomes a matter of time and temperature with very little stress needed. Shot peened springs should be stress relieved before use at higher temperatures since they tend to lose stability due to stress relief. However, if endurance strength is required, a moderate stress relieving temperature (480-500 Deg. F) is used. Set can be controlled in springs in several ways. In one method called "hot setting", the spring is held solid on an arbor at as high a temperature as will permit it to return to the required length. This permits the spring to become stabilized. In heat setting, as distinct from hot setting as past described, hardened steel springs are placed in dies and held under pressure at the drawing temperature. This is designed to maintain the proper shape despite changes which occur in quenching. Shot peening is used to increase the fatigue life of springs. Today there is no test to determine the degree and completeness of peening except by fatigue testing. The Almen is a fine machine check but nothing more. There is no condition between the almen strip arc heights and the endurance limit of the spring. Shot peening, since it is a mechanical working process, can be modified by heat treatment. In fatigue tests at various temperatures there was little change between life of peened and unpeened springs up to 250 Deg. F. Above 250 Deg. F., the peened spring became progressively worse until, at 400 Deg. F. the original increase in endurance limit of 35,000 PSI had been reduced to 10,000 PSI.