Correlations between microstructure and mechanical properties during PWHT of TMCP steel weld HAZ

Summary

This paper describes an investigation of the correlation between HAZ microstructures amd mechanical properties in the post‐weld heat treatment (PWHT) of two types of thermomechanical control process steels (TMCP steels), especially the relationship between the stress relaxation behaviour and high‐temperature deformability. Simulated weld heat treatment was performed with a welding thermal cycle simulator at a maximum temperature of 1623 K. PWHT was performed at a heating rate of 55.6 K/ksec. The mechanical properties in the PWHT process were evaluated by rising‐temperature constant‐strain rate tests and rising‐temperature constant‐load tests. The effect of PWHT in reducing ductility is discussed from the perspective of the precipitation behaviour of intergranular and transgranular carbides and the associated deformability of the matrix in each HAZ structure. The results obtained may be summarised as follows:

1. The results obtained during measurement of the stress relaxation behaviour in the rising‐temperature constant‐strain rate tests suggest that the bainite structures of both steels clearly show more stagnation or delay in their stress relaxation behaviour than the other HAZ structures at a PWHT temperature above 600 K. This implies that the matrix is resistant to softening. The non‐AcC type steel also exhibits more stagnation in the higher temperature range under the effect of alloy carbide precipitation at the grain boundaries than the AcC type steel.

2. The results obtained in the rising‐temperature constant‐load tests run to determine the high‐temperature strength and inherent deformability of the HAZ structure suggest that the bainite structures of both steels tend to lose more ductility than the other HAZ structures, having a reduction of area of 35% at a fracture temperature of 850–900 K. The non‐AcC type steel also exhibits a greater loss of ductility in all HAZ structures than the AcC type steel.

3. The results of the TEM observations made to determine the causes of this ductility loss suggest that a difference in the carbide precipitation behaviour near the grain boundaries in each HAZ structure in the PWHT process affects the plastic deformability of the matrix, and that the trend of reducing plasticity differs in each HAZ structure. These trends are more pronounced in the non‐AcC type steel containing alloying elements with a strong carbide‐producing tendency, such as e.g. Nb, Ti, etc.

4. All HAZ structures of the AcC type steel show a trend of reducing ductility at a fracture temperature of 850–900 K. This feature is not found in conventional carbon steels with an identical composition and may well be due to the fact that this temperature range corresponds to the ductile‐brittle transition range. It is necessary to resort to a method of fabrication able to reduce the hardened structures as far as possible during welding, i.e. to ductility reducing counter‐measures in the PWHT process, such as e.g. welding heat input control, preheating, etc.

5. To evaluate the ductility and brittleness of steels, it is important to gain a good understanding of their plastic deformability, and the paper proposes a method for evaluation of the ductility of the TMCP steels on the basis of the relationship between the amount of displacement produced in the rising‐temperature constant‐strain rate tests and the plastic deformability of each HAZ structure in the PWHT process as obtained in the rising‐temperature constant‐load tests. This method enables the risk of cracking and degree of embrittlement to be identified and proves effective in practical applications.