2002 年 2002 巻 47 号 p. 43-61
In the first half of 1980s, The Japan Society for Spring Research held the joint committee which researches shot-peening condition for small springs. This committee proposed optimum shot-peening conditions for steel grades and wire diameters. Resent years, strength of spring material has been targeted to be higher and higher levels. Shot-peening technology has been improving, and new shot-media and multi-shot-peening technology have become to be employed. Therefore, the joint research was needed for high strength spring. Under these circumstances, a research committee for optimizing a shot-peening process was established in September 1996. In this committee, the experimental shot-peening conditions were widely chosen in the possible range of manufacturing parameter. Single shot-peening, double shot-peening, stress shot-peening and hardness of spring were studied. The effect of each shot-peening process on fatigue strength were considered in relation to surface compression stress and surface roughness. For control of shot-peening condition, effectiveness of a harder Almen strip, relation between X-ray radiation field size and wire diameter, and the correction method of residual stress were studied. The results are summarized as follows.
(1) For high strength spring, a harder Almen Strip with hardness of 58HRC, thickness 1.3mm, is proposed.
(2) Research on suitable shot-peening conditions for high strength spring:
(a) The difference of torsional and rotary fatigul test is negligible.
(b) In the case of single shot-peening, higher surface residual stress and smaller surface roughness induced high strength of fatigue
(c) In the case of double shot-peening, harder and finer shot indicated higher strength of fatigue.
(d) Stress shot-peening could not be better than double shot-peenig with higher and finer shot.
(e) Double shot-peening with higher and finer shot indicated the best fatigue strength.
(3) Analysis of fatigue phenomenon for a shot-peened spring
(a) Compression residual stress on surface is the most effective for improving fatigue strength.
(b) Stage I type crack first initiates and propagates and then stage I crack transfers into stage II type crack. Magnitude of stage I crack is almost equal to depth of compression residual stress.
(4) X-ray stress measurement:
(a) In the case of coil spring, X-ray radiation field size should be smaller than 1/2∼1/3 of wire diameter.
(b) By using FEM method, Heyn's correction equation is practically useful for correction of residual stress.