THE IMPORTANCE OF HEAT TREATMENT （热处理的重要性）.pdf

THE IMPORTANCE OF HEAT TREATMENT ENGINEERING ABSTRACT Although the major codes for fabrication, generally give clear instructions on the pre- requisites for heat treatment, the implications of carrying out such heat treatment can be significant, and is easily overlooked by the less experienced contractor. This paper looks at the most common mechanical problems associated with heat treatment, and the necessary mechanical solutions to ensure a successful heat treatment. Author Mr K J Bolton Stork Technical Services (CooperHeat) Unit 21-24 Slaidburn Crescent Southport Merseyside PR9 9YF United Kingdom August 2012 1 THERMAL EXPANSION The source of many problems associated with heat treatment arises from the effects of thermal expansion. Although the principal of thermal expansion is readily understood, problems can arise as a result of thermal expansion, but these are not always so apparent. The thermal expansion of carbon/ low allow steels ranges from 11.5x10-6m/m°C at ambient, to 14.8x10-6m/m°C at 600°C. An easier number to work with for estimating purposes is 8mm per metre for carbon steels at 600°C. Stainless steels have significantly higher rate of rates of expansion, -approximately 30% greater than that of carbon steel; this can cause problems where stainless steel components are included in carbon steel heat treatments. The forces generated by thermal expansion of a constrained component can be massive, and if constraints are not removed, then both distortion, and / or high residual stresses are likely following heat treatment. The following example highlights this: Pipework – the effects of constrained ends Many codes make reference to thermal gradients, and the ‘harmful effects’ that may r