K. Mariappan

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.

Influence of Varying W and Ta on Low Cycle Fatigue and Creep–Fatigue Interaction Behavior of Reduced Activation Ferritic/Martensitic Steels

The objective of the present paper is to provide first hand experimental data and analysis on the low cycle fatigue (LCF) and creep–fatigue interaction (CFI) performance of different chemical compositions of reduced activation ferritic–martensitic (RAFM) steels. As a part of the development programme, LCF and CFI experiments were conducted on different RAFM steels with varying W and Ta obtained in the normalized and tempered condition. Effect of varying Ta and W upon the resultant LCF and CFI life seemed to be interrelated and an optimum combination of both W and Ta worked out to be the best for CFI life. Stress relaxation that occurs under the application of hold has unique information of the underlying deformation and damage in the RAFM steels. It has been observed that stress relaxation is directly related to the amount of plastic strain accumulated, inversely related to CFI life and strongly dependent on chemical composition. Hence it can be used to explain the deformation and damage accumulation in ferritic–martensitic steels. The plastic strain accumulated during stress relaxation is accommodated as microstructural coarsening. EBSD based microstructural characterization reveals that the recovery process is closely related with the chemical composition and various test parameters. Breakage of protective chromia layer accounts for the more damaging effects of oxides under compressive hold.

Low Cycle Fatigue and Creep-Fatigue Interaction Behaviour of Reduced Activation Ferritic Martensitic (RAFM) Steels with Varying W and Ta Contents

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for the test blanket modules of ITER. Several degradation mechanisms such as thermal fatigue, low cycle fatigue, creep fatigue interaction, creep, irradiation hardening, swelling and phase instability associated irradiation embrittlement must be understood to estimate the component lifetime. The current work focuses on the effect of tungsten and tantalum on low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of four RAFM steels with varying W and Ta contents. Total strain controlled LCF experiments were performed under various strain amplitudes in the range +0.25% to +1% and temperatures (300 K to 873 K) in air at a constant strain rate of 3×10-3s-1 using a servo hydraulic fatigue testing system. CFI experiments were carried out at total strain amplitude of +0.6% and by applying strain hold of different durations (10 min and 30 min) in peak tension and peak compression. Both LCF and CFI life of the RAFM steels improved with the increase in tungsten and tantalum contents. Based on the amount of softening during continuous cycling, tungsten content was optimized at 1.4 wt. % and the tantalum content at 0.06 wt%. Stress relaxation obtained during creep-fatigue interaction studies showed close relation with the chemical composition of the RAFM steels. Other damaging parameters influencing fatigue life were dynamic strain ageing (DSA) occurring in the intermediate temperature regime and oxidation at elevated temperatures. Keywords: RAFM steel, low cycle fatigue, dynamic strain ageing, creep-fatigue interaction, oxidation