As part of a study for the mechanical properties of spring steel, investigation was made on the effects of sorbitic microstructure, containing proeutectoid ferrite or hardness gradient, on the mechanical properties of C and Mn-Cr steels.
Specimens were prepared mainly from spring steels and also from structural steels or casehardening steels for a supplementary pourpose. They were quenched & tempered, highfrequency induction-hardened & tempered, or quenched after carburizing & tempered, and divided into following two groups:
A. Specimens, heat-treated to the spring hardness (Hv 400 approx.) throughout the diameter and containing 0--35% of proeutectoid ferrite.
B. Specimens, heat-treated to the spring hardness at the surface but to the lower hardness (Hv 210-400) at the center.
Results obtained after mechanical tests were summerized as follows.
1. The proeutectoid ferrite shows no clear effect upon the tensile strength.
2. The elongation, reduction of area, Charpy impact value, proportional limit in twisting, proportional limit in bending, and repeated impact value are lowered by the existence or increase of proeutectoid ferrite.
The degree of lowering is more remarkable in C steel than in Mn-Cr steel.
3. The fatigue limit in rotary bending is also decreased by the proeutectoid ferrite, but the behaviors in each kind of steel were not revealed distinctly.
4. When the yielding shear stress curve along the radius of specimen, which is determined by the hardness gradient curve, is crossed over by the theoretical load stress line, the maximum shear stress and also the reversed twisting fatigue limit are lowered.
5. To improve the proportional limit in twisting or reversed twisting fatigue limit for the steel bar having hardness gradient, the depth of hardened layer or core hardness must be increased. The relationship between those is given in a following formula:.
Where τmax: proportional limit of bar in twisting
τp: proportional limit of core in. Ntwisting
d: diameter of bar
t: depth of hardened layer.