M. Nani Babu

Effect of test temperature on fracture toughness of modified 9Cr–1Mo steel

Abstract The quasi-static fracture behaviour (J–R curves) of modified 9Cr–1Mo (P91) steel was studied. The J–R curves were established at 298, 653, 823 and 893 K, and fracture toughness J0·2 at 0·2 mm of crack extension was determined. The value of ∼J0·2 at 653 K was lower compared to that at 298 K followed by increases in J0·2 values at 823 and 893 K. The decrease in J0·2 at 653 K can be attributed to the influence of dynamic strain aging. At 893 K, a significantly higher (more than 200%) J0·2 was observed, since plastic deformation of the net section, rather than crack growth, occurred in this condition.

Fatigue Crack Growth Behavior of Ferritic and Austenitic Steels at Elevated Temperatures

For the integrity assessment and damage-tolerant design of components operating at elevated temperatures under cyclic loading conditions, evaluation of fatigue crack growth (FCG) properties in the threshold and Paris regimes is important. The FCG behaviors of a modified 9Cr-1Mo steel (P91) and SS 316L(N) for applications in the prototype fast breeder reactor to be commissioned shortly at Kalpakkam, India have been studied extensively in our laboratory over a wide range of temperatures. Some aspects of high-temperature FCG observed in these investigations will be reviewed in this paper. It has been observed that for many engineering alloys, dynamic strain aging plays an important role in the temperature-dependent variations in deformation and fracture behavior, including the FCG parameters. Another important aspect in deciding the FCG behavior is the crack closure behavior, the mechanisms of which may vary with temperature. Effect of crack closure and DSA on the FCG properties of P91 steel and SS 316L(N) welds will be discussed. The DSA mechanism has been identified from the activation energy for the process determined from the temperature dependence of the crack tip strain rates. During FCG of engineering materials, there can be stress shielding at the crack tip because of various factors, like crack bridging, branching, closure, etc. Effect of stress shielding at the crack tip at different conditions was quantified using a procedure incorporating the inter relations between compliance, crack length, and stress intensity factor. The paper will summarize the results of the above studies.

High-Temperature Fatigue Crack Growth Behaviour of SS 316LN

FCG behaviour of SS 316LN steel is evaluated in the temperature range 300–823 K. While there is a general increase in FCG rate with temperature and load ratio, specifically at low applied stress intensity factor ranges, for intermediate temperatures (623–723 K) and applied stress intensity factors (15–25 MPa m1/2), a cross over in the crack growth rate is observed. The Paris exponents for different temperatures varied between 2.4 and 3.7. The variations in the crack growth rates are examined by considering the crack closure and dynamic strain ageing (DSA) effects. Attempts have been made to rationalize these variations with the temperature dependence of Young’s modulus and yield strength. The stress intensity factor range normalized with yield strength gives a better correlation with FCG rates at different temperatures. The unified data for all the temperatures was fitted to a Paris-type correlation, viz., \( \frac{{{\text{d}}a}}{{{\text{d}}N}} = C \cdot \left( {\frac{\Delta K}{{\sigma_{\text{ys}} }}} \right)^{m} \) with C = 4.5 nm/cycle and m = 2.73.

Effect of Rolling Temperature on Fracture Properties of INRAFMS at Different Temperatures

Two heats of Indian reduced activation ferritic / martensitic steel have been produced with similar chemical composition and identical process parameters except the start and final rolling temperature referred as P950 and P1050, respectively. Fracture behaviours of both the heats have been evaluated, and it was found that quasistatic fracture properties are better for P1050 than P950. An attempt is made to establish the reasons for the observed behaviour through optical, scanning electron microscopic, electron backscatter diffraction and transmission electron microscopic studies. These variations are attributed to the difference in rolling temperature of both heats. P1050 meets the acceptance criteria as per Indian specifications, and it compares favourably with the international grades of equivalent RAFM steels.

Influence of Long Term Ageing on Fatigue Crack Growth Behavior of P91 Steel at Different Temperatures

Fatigue crack growth behavior (FCG) of Modified 9Cr-1Mo (P91) steel has been evaluated at 300, 653 and 823 K in normalized and tempered (unaged) and in subsequently aged at 823 K for 20000 h (aged) condition in both Paris and threshold regimes. The effects of test temperature and ageing on the FCG resistance of the material have been rationalized by examining the behavior using different measures of driving forces, namely, the applied and effective stress intensity factor ranges ΔK and ΔK eff , and the crack tip stress intensity factor range ΔK tip,eff that takes in to account the stress shielding effects also. The variations in the extent of crack closure in the threshold regimes and the crack path tortuousness at different temperatures indicated that in addition to plasticity, roughness induces closure at 300 and 653 K, while at 823 K possibly oxide induced closure too takes place. Crack growth rates for the aged material are higher than that of unaged P91 steel for all the temperatures examined, irrespective of the choice of driving force and is attributed to the loss in strength as well as formation of embrittling Laves phase.