MEASURING THE EXTENT OF STRESS RELIEF
Until recently there was no reliable method for the precise measurement of residual stress, that not only originated from welding, but also from forging, cold drawing and other types of metal working. Now, with the use of diffractometry with x-rays, the problem has been solved.
In the past, the only way of checking if residual stress had been reduced to an acceptable level was by analogy with hardness. It is a well known fact that materials get harder when submitted to stress. Experience acquired over the years, upon which the applicable standards are still based, demonstrated that if the hardness measured after stress relief had been performed was lower than a given empiric value, the treatment had been successful.
This condition was especially important if the weld was to be in contact with corrosive environments, as is the case of the chemical industry. It is well known that, depending on the environment, metals show less corrosion resistance if submitted to residual stress.
This fact is taken into account in ASME/ANSI B 31 Code for Pressure Piping. Section B 31.3, which is used in process industries such as oil refineries and chemical plants, states the maximum allowable hardness in welds after stress relief. This requirement does not exist in Section B 31.1, which is used in power plants, where the possibility of corrosion is much less.
Until very recently, diffractometry by x-rays presented a serious operational problem. The equipment was too large to be taken to the point of use, and in many cases the weld could not be taken to the equipment. This situation has been overcome by the development of equipment small enough to be moved from one place to another. The applicable codes, however, continue to consider Hardness as the parameter ruling the approval of stress relief, and this is why we have adopted it in the Experimental Section.