Residual stress and distortion

Heat treatment is related with stress-and-strain problem, such as residual stress, quenching crack and distortion/deformation. Residual stress is defined as the self-equilibrating internal or locked-in stress remaining within a body with no applied force, external constraint, or temperature gradient. (Ref.1)

Residual stress can be classified as two kinds:
1.Macro- or long-range is the first order stress, which represent average stress across all the phase transition in the polyphase material. Compared with grain size of material, macroresidual stress act over large regions. Traditionally, engineers have been only considering this kind of residual stress when designing the mechanical components.

2.Microresidual stress or tesselated stress or short-range stress is a second-order or texture stress, which is associated with lattice defects (such as vacancies, dislocations, and pile-up of dislocations) and fine precipitates (for example,martensite) (Ref.1) Macro-residual stress is the average stress across one grain or part of one grain. This information is indispensable in study of the material deformation.

These two residual stress can be classified further as tensile or compressive stress near surface or in the body of the material. This article briefly discusses the effect, control and measurement of long-range one.

Residual stress can cause dimensional changes and resistance to crack initiation. Dimensional changes occurs when  stress or part of the stress in the body of the material is eliminated.In terms of crack initiation,whether the residual stress is beneficial or detrimental depends on whether it is tensile stress or compressive stress.

Compressive stress. Because residual stresses are algebraically summed with applied stresses, residual compressive stresses in the surface layers are generally helpful because the built-in compressive stresses can reduce the effects of imposed tensile stresses that may produce cracking or failure. Therefore compressive  stress contributes to the improvement of fatigue strength, resistance to stress-corrosion cracking, and an increase to the bending strength of brittle ceramics and glass.

Tensile residual stress on the part surface, on the other hand, is undesirable because they increase the stress level in short time; may cause unpredicted stress-corrosion crack (because of the effect of stress and environment),fatigue failure,quenching crack and grinding checks at the low external stress, and may also reduce the fatigue life and strength of the parts. In this case, the extent of the residual stress may be closer or even larger than the strength of the material.

It is worth noting the effect of tempering on the residual stress levels. The tempering temperature should be accomplished at 150°C to maintain the 50-60% of the residual stress after quenching for the reason that higher tempering temperature will largely reduce the surface compressive stress. However, higher stress relief temperature (~600°C)is used in mechanically deformed parts or the parts with surface tensile stress. Alternatively, serious residual stress can be avoided by gentle grinding on the surface.

Measurement of residual stress

There are two methods in the measurement of residual stress:
1) Destructive method is old but accurate, and is applied in confined situation at site. Some drawbacks of this methods include tedious, time consuming, expensive and can only be used to macro-residual stress.
2) Nondestructive method mainly includes X-ray diffraction, neutron diffraction, ultrasonic, and magnetic testing.

In the next article, we will continue talking about the relationship between residual stress and distortion.

after heat treatment.

REFERENCES

1. Anil Kumar Sinha, Bohn Piston Division , ASM Handbook, Volume 4: Heat Treating ASM Handbook Committee, p 601-619