Effect of microstructure on elastic property at high temperatures in ferritic heat resistant steels

Elastic moduli were evaluated by tensile and abrupt stress loading tests at high temperatures in the JIS STBA24, ASTM A542 and ASME T91 steels to clarify the influence of microstructure on estimation of the elastic modulus. In the JIS STBA24 steel of ferrite-pearlite structure, the elastic moduli evaluated from conventional tensile tests are in good agreement with that obtained from ultrasonic pulse method over a wide temperature range. On the other hand, in the ASTM A542 and ASME T91 steels of tempered martensitic lath structure, the elastic moduli evaluated from conventional tensile tests abruptly decreases at temperatures higher than 773 K, in contrast with the elastic moduli obtained by ultrasonic pulse method. Anelastic deformation occurs at high temperatures under applied stress in the ASTM A542 and ASME T91 steels due to elastic bending of the lath boundary. In the steels of martensite, elastic moduli estimated by instantaneous strain, which contains no anelastic deformation upon abrupt stress loading agree with those obtained by ultrasonic pulse method. The elastic modulus does not depend on the strain rate at 873 K in the JIS STBA24 steel. On the other hand, the elastic modulus decreases with decreasing strain rate at 873 K in the ASME T91 steel due to the anelastic deformation during tensile tests.