L. Allais

Modelling creep damage in heat affected zone in 321 stainless steel. Part I: Quantitative study of intergranular damage

Abstract During service at high temperature, stabilized stainless steels are well known to present a serious form of intergranular cracking in the Heat Affected Zones (HAZ) referred to as the reheat cracking phenomenon which is very deleterious for the structural integrity of welds made in this steel. The purpose of the present study is to present quantitative results on mechanical behaviour and the damage at high temperature of simulated HAZ which show different sensitivities to intergranular cracking. Creep laws are determined and an intergranular damage model is identified using the local approach to fracture based on mechanical tests performed at 600 °C and metallurgical observations on smooth specimens and two types of notched specimens.

Modelling creep damage in heat affected zone in 321 stainless steel. Part II: Application to creep crack initiation simulations

Abstract Stabilized stainless steels like AISI 321 are well known to present a serious form of intergranular cracking in the HAZ referred to as reheat cracking. The purpose of the present part is to validate the creep damage model identified using the local approach methodology on notched bar creep tests (Part I) by checking if these damage laws are able to give accurate fracture predictions. The comparison between experimental data obtained at 600°C and numerical simulations is made for five different conditions with three different simulated HAZ and on two specimen geometries including CT specimen tested under stress relaxation conditions and sharply notched thin wall tubular specimens tested in torsion (Mode II loading). Both of these specimens have not been used for the identification of the damage model coefficients.

Microstructural creep damage in welded joints of 316L stainless steel

The present work has been undertaken to study creep damage in welded joints. The complex dual phase microstructure of 316L welds are simulated by manually filling a mould with longitudinally deposited weld beads. Most of the moulded specimens were then aged for 2000 hours at 600°C. High resolution scanning electron microscopy was extensively used to examine the microstructure of the welded material before and after ageing. Columnar grains of austenite constitute a matrix in which thin dendrites of δ-ferrite can be found. The ageing generates the precipitation of carbides, resulting in less transformation in the material. Smooth and notched creep specimens were cut from the mould and tested at 600°C under different stress levels. The creep life of the simulated welded material is shown to be lower than that of the base material. Microstructural observations reveal that creep cavities are preferentially located along the austenite grain boundaries. This analysis of intergranular damage on test specimens is ...

AGEING AND CRACKING OF SIMULATED HEAT AFFECTED ZONES IN 321 STAINLESS STEEL

The heat affected zone (HAZ) of stabilised austenitic stainless steel welds may exhibit a serious form of intercrystalline cracking during service at high temperature. This type of embrittlement, well known as stress relief cracking, is related to thermal ageing: a fine and abundant intragranular Ti(C,N) precipitation appears near the fusion line during service at high temperature and modifies the mechanical behaviour of the HAZ. To analyse this embrittlement micro mechanism and to assess the lifetime of real components, different HAZ were simulated by various solution heat treatments, cold rolling and ageing conditions. The mechanical behaviour of these resulting materials was investigated using creep and tensile tests on smooth bars. Then, creep tests were carried out and simulated on notched bars by finite element calculations. A damage model was identified from intergranular damage measurements made on notched specimens and compared with calculated mechanical fields. Further tests on fatigue precracke...