Effect of high-temperature thermomechanical treatment (HTMT) on the fatigue strength of high-carbon steels

Summary Fatigue tests on steels 9KhS and ShKh15 revealed anisotropy in fatigue properties of these steels after HTMT.In asymmetric cycling, the fatigue limit depends to a large extent on the magnitude and direction of the mean stress of the cycle. Superimposition of a direct load (in the entire range of ₀ studied) increases the fatigue limit (sometimes by 100%), of steel subjected to HTMT; when an inverse load is superimposed, the fatigue limit after HTMT is lower than after a conventional heat treatment.The increase in fatigue strength of high-carbon steels after HTMT is achieved in the first instance as a result of the directional congruence of weak regions and external stresses. This effect becomes particularly evident under conditions of asymmetric stress cycling with a superimposed static load.     

The fatigue strength of steels containing small holes under out-of-phase combined loading

A criterion is presented for the prediction of the fatigue strength of defect-containing specimens subjected to combined axial and torsional loading at an R of −1. The criterion is applicable to loading situations where there is an arbitrary phase difference between the axial and torsional loadings. In order to check on the validity of the criterion, fatigue tests were performed using specimens of annealed carbon steel as well as of heat-treated alloy steel. The specimens contained a surface hole, which was either 100 or 500 μm in diameter. Good agreement between predictions and experimental results was obtained for both in-phase and out-of-phase loadings.     

Effect of heat treatment on fatigue crack growth in chain steels

Abstract

The influence of tempering temperature, stress ratio, and prior strain on fatigue crack propagation in a low-alloy chain steel has been investigated. At small stress ratios (R=0·1) tempering above 400°C is beneficial, resulting in higher threshold levels and slower growth rates in the initial growth regime. Thereafter, crack growth is independent of tempering temperature, as it is over the entire growth period under a high mean stress (R=0·5). Prior strain produces a slower growth and higher thresholds at R= 0·1. Intergranular fracture is common and is a function of stress intensity range and tempering temperature. It is concluded that residual stress effects, rather than microstructural effects, account for the experimental observations. In particular, the existence of a tensile residual stress during initial growth and a crack closure stress greater than the minimum applied stress level are proposed.

MST/672     

Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels

A new alumina-forming austenitic stainless steel with greatly improved high-temperature oxidation resistance and strength was developed via alloying 3.0 wt.% Al in the Fe-25Ni-18Cr based alloy. Continuous, stable and exclusive alumina scale was formed in either dry air or air with 10% water vapor mixed environment at 800 °C. The long-term high-temperature oxidation performance is appreciably enhanced which is associated with the high density of the B2-NiAl precipitation phase maintaining the Al₂O₃ surface layer. Moreover, when tested at 750 °C in dry air environment, the new steel showed high yield and fracture tensile strength of 310–335 and 480–500 MPa, respectively.     

Strain-amplitude dependent fatigue resistance of low-alloy pressure vessel steels in high-temperature water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in 561 K air and water over a wide strain amplitude range. It was found that fatigue resistance of the steels was enhanced in high-temperature water relative to high-temperature air under the low strain amplitude conditions (<0.3%) or in the high cycle regime (>2 × 10⁴ cycles), while it was remarkably degraded in high-temperature water under the higher strain amplitude conditions. Fatigue cracking and fractographic features suggested that effects of hydrogen be involved in the present corrosion fatigue process in high-temperature water. Possible environmentally assisted cracking mechanisms are discussed.     

Mechanism of corrosion fatigue crack propagation in high strength steels

The crack propagation velocity in corrosion fatigue $\text{(d a/d N)}{}_{\mathrm{c}}$ were measured on the Ni-Cr-Mo steel quenched and tempered at 473 or 773 K.The steel with high sensitivity to delayed failure reveals the largest $\text{(d a/d N)}{}_{\mathrm{c}}$ under square load and the smaller $\text{(d a/d N)}{}_{\mathrm{c}}$ under positive saw tooth load. The frequency dependency of crack propagation characteristics indicates that the interaction between hydrogen atoms and the cyclic moving of triaxial position at crack tip acts an important role in the crack propagation mechanism, i.e. hydrogen concentration process controls the crack propagation of the steel.The steel with low susceptibility to delayed failure reveals, on the other hand, the largest $\text{(d a/d N)}{}_{\mathrm{c}}$ under the positive saw tooth load but the smallest $\text{(d a/d N)}{}_{\mathrm{c}}$ under the square load, i.e. the stress increasing time is important and the hydrogen invasion process is the controlling factor for the crack propagation.     

Near-threshold fatigue crack propagation of several high strength steels

In support of Douglas Aircraft Co. structural damage tolerance analysis, crack propagation rates were measured in humid air from 10^?8 to 10^?3 mm/cycle of several high strength martensitic steels at stress ratios of $\text{R = 0.1}$ and $\text{R = 0.5}$. Increasing the stress ratio was found to decrease the threshold stress intensity. Increasing the tensile yield material strength in these quench and tempered steels resulted in a nearly linear inverse relationship with the threshold stress intensity. The threshold stress intensity appeared to be independent of the fracture toughness of the alloy steel.     

Early detection and monitoring of fatigue in high strength steels with MWM-Arrays

Fatigue monitoring of cyclically loaded shot peened high-strength steel components can be accomplished via magnetic permeability measurements during laboratory tests or in service. These measurements can be performed either continuously using permanently mounted Meandering Winding Magnetometer Arrays (MWM^®-Arrays) or intermittently with scanning MWM-Arrays. The results obtained to date suggest that MWM-Array permeability measurements can provide early detection of fatigue damage in steels before conventional methods can detect any changes. This has been demonstrated to be particularly significant in the presence of high compressive stresses introduced by shot peening. One of the fatigue tests was suspended when accelerating changes in local permeability were detected. Examination of the fatigue specimen in a scanning electron microscope detected only a few relatively small cracks, e.g. 50–200 μm long at the surface. Fractography, however, revealed significantly longer cracks. For the same specimen, conventional eddy current and ultrasonic testing failed to provide any indications of cracks, and fluorescent liquid penetrant detected only an inconclusive spot indication. This paper provides a comparison of the permeability changes and fractography data with a fatigue crack growth curve based on a FASTRAN analysis accounting for residual stresses from shot peening. A comparison of the experimental data and crack growth analysis results suggested that MWM-Array magnetic permeability measurements may detect cracks in the compressive stress field when they are about 50 μm deep.     

Development of mechanical properties of structural high-carbon low-alloy steels through modified heat treatment

The modified heat treatment, which produces a mixed structure of martensite and lower bainite through short-term isothermal transformation at just above the martensitic transformation temperature,M _s temperature, followed by oil quenching (after conventional austenitization), has been applied to three high-carbon low-alloy steels with different levels of nickel and chromium contents at similar molybdenum levels, in which carbon was allowed to replace relatively expensive additions of nickel and chromium, for their ultra-high strength application. The significant conclusions are as follows: an ultra-high strength steel of 1900 M Pa yieldstress grade with a high toughness level can be obtained when about 60 vol % lower bainite is associated with 473 K tempered martensite of 0.60% C-1.80% Ni-0.80% Cr-0.25% Mo steel. If approximately 25 vol % lower bainite appears in 673 K tempered martensite of the steel, a 1700 M Pa yield-stress grade steel with high toughness and moderate ductility levels can be attained. However, alloying nickel is essential to some extent for development of the mechanical properties with the modified heat treatment suggested in the present work.     

X20CrMoV12.1钢的组织结构和缺口疲劳强度

用扫描电镜和透射电镜观察X20CrMoV12.1耐热钢在545℃长期运行后的微观组织结构变化,研究了不同运行时间后材料的缺口疲劳强度和硬度.结果表明:经过高温长期运行后,X20CrMoV12.1钢的组织明显退化,其显微组织由原来的马氏体板条结构和弥散分布的细小碳化物逐步转变为亚晶粒和分布在晶界上的粗化球形碳化物组织;随着在高温下运行时间的增加,耐热钢的显微硬度和疲劳强度逐渐降低.其原因是基体中固溶体主要合金元素Cr,Mo,V发生贫化,固溶强化效果降低,沿晶界分布的粗化球形碳化物弱化了材料晶界强度,从而导致X20CrMoV12.1耐热钢高温服役寿命下降.