• Stress relief heat-treatment is defined as the uniform heating of a structure to a suitable temperature, holding at this temperature for a predetermined period of time, followed by uniform cooling (uneven cooling may result in additional stresses).

• Stress relief heat treatment is usually performed below the critical range so as not to affect the metallurgical structure of the work.

• The percentage relief of internal stress depends upon the type of steel (its yield strength). The effects of varying time and temperature are shown in Fig. 5.11.

the weld bead is heated to 175O-205OC while the weld itself is relatively cool. This causes thermal expansion in the base metal and a reciprocal tensile stress in the weld beyond the yield. When the metal cools and contracts, the stress falls below the yield. When the process is used properly a partial reduction in the longitudinal stresses of butt welds is achieved.

5.3.4 Peening

Peening has been used by the welding industry for over 35 years, but the code requirements and regulations governing this procedure have been based on opinion rather than on scientific data because there has been no practical method for measuring the effect of peening.

Various specifications and codes require that the first and last layers of a weld should not be peened.

The results of laboratory tests conducted by American Bureau of Shipping and explo­sion tests by the Naval Research Laboratory confirm the requirement prohibiting the peening of the first and the last layers.

In conducting peening, the following special precautions may be necessary:

(1) Work hardening should be considered when certain AISI 300 series steels are involved.

(2) Hot shortness may preclude hot peening of certain bronze alloys.

(3) AISI 400 series steels have relatively poor notch ductility in the as-welded condition. Utmost care should be exercised if peening is attempted.

(4) The relative elongation values for ductility of welds and metals should be considered before employing the peening process.

Peening equipment should be selected with care The hammer, pneumatic tools, and so forth should be sufficiently heavy for striking force to be effective without producing excessive work hardening, but not so heavy as to involve bending moments or produce cracks in the weld.