Crack propagation in gaseous atmospheres

Environmentally-assisted cracking is studied for an Al-Zn-Mg alloy in different gaseous atmospheres. A sophisticated experimental installation with an ultra-high-vacuum chamber (10^-5) is used to maintain and monitor a high-purity atmosphere surrounding the specimen. The experimental data show that water vapor is the main factor responsible for the enhancement of the crack growth rates as compared with those in the high vacuum. Embrittlement is attributed to the influence of hydrogen produced as a result of the surface reaction of water vapor with the fresh metal surface near the crack tip. Departamento de Ciencia de Materiales, Escuela de Ingenieros de Caminos; Universidad Politecnica de Madrid. Madrid, Spain. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 65–70, July–August, 1998.     

Carbide precipitation in some secondary hardened steels

The precipitation of alloy carbides in three commercial quenched and tempered martensitic steels has been studied using phase stability calculations, microscopy, microanalysis and dilatometry. The sequence of carbide stability as estimated using phase stability calculations is found to be consistent with the microstructural observations, if it is assumed that the equilibrium M₂₃C₆ phase is suppressed by kinetic considerations. One interesting result is that the molybdenum-rich phase M₂C is neither predicted nor observed in any of the alloys, all of which contain substantial quantities of molybdenum. Other results include dilatometric experiments which enabled the measurement of transformation temperatures, which are compared against theoretical values. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of grain size on crack propagation of high strength steel in gaseous hydrogen atmosphere

Abstract

The effect of prior austenite grain size on the crack propagation behaviour of tempered martensitic steels having tensile strength of about 2 GN m^?2 was studied in hydrogen gas at pressures in the range from 98 to 784 kPa using modified compact tension specimens. The crack propagation rate da/dt in hydrogen decreased as the prior austenite grain size increased from 45 to 450 μm. The dependence of da/dt on hydrogen pressure at a given applied stress intensity was examined. The permeation of hydrogen from the crack tip surface was estimated to decrease with increasing grain size. However, the fractographic study suggested that the degree of embrittlement of grain boundaries increases with grain size. Consequently, the inverse effect of grain size on da/dt may be caused by a decrease of the average concentration of hydrogen along grain boundaries at the crack tip with increasing grain size.

MST/1060     

Mechanical properties of martensitic steels in gaseous hydrogen

We present the regularities of hydrogen degradation of 03Kh12N10MT, 15Kh12N2MFAV and 13Kh11N2V2MF steels under a pressure of 30 MPa within the temperature range of 293–673 K. The minimum values for plasticity, low-cycle fatigue, and static crack resistance, which do not decrease with an increase in pressure of hydrogen atmosphere and content of the absorbed hydrogen, are found. The difference between temperature dependences of the coefficients of influence of hydrogen on static and cyclic crack resistance of martensitic steels with various content of austenite is established. The main fractographic features of the influence of hydrogen on the micromechanism of fracture of steels under different types of loading and temperatures are determined.     

Crack propagation behavior of solution annealed austenitic high interstitial steels

Austenitic stainless steels provide a beneficial combination of chemical and mechanical properties and have been used in a wide field of applications for over 100 years. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. But the solubility of N within the melt is limited and can be increased in substituting Ni by Mn and melting under increased pressure. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS). Within the solution annealed state strength and ductility of AHIS is comparable or even higher of those of AHNS and can be further improved by cold working. Unfortunately the endurance limit does not follow this trend as it is known from cold-worked austenitic CrNi steels. This is due to the differences of the slip behavior which is governed by the stacking fault energy as well as other near field effects. Construction components operating under cyclic loads over long periods of time cannot be considered being free of voids or even cracks. Thus the crack propagation behavior is of strong interest as well. This contribution presents the tensile, fatigue, crack propagation and fracture toughness properties of AHNS and AHIS in comparison to those of CrNi-steels. The differences are discussed in relation to microstructural characteristic as well as their alterations under cyclic loading.     

Kinetic aspects of slow crack growth in the gaseous hydrogen embrittlement of steels

An explanation of the slow crack-growth phenomenon in gaseous hydrogen embrittlement is suggested on the basis that chemisorption is the rate-limiting step. The basis of the analysis is the existence of a mobile adsorbed species which is a prerequisite to the occurrence of slow crack growth. The disappearance of the mobile species with increasing temperature results in the observed crack-velocity dependence on temperature. The analysis is able to account qualitatively for the observed dependence of crack velocity with pressure in the different temperature regions of crack growth.     

Crack arrest in steels

This paper examines 3 theories that have been used to characterize the arrest capabilities of steels and structures: (1) The static analysis, arrest toughness ($\text{K}{}_{\mathrm{Ia}}$) theory; (2) The dynamically loaded/stationary crack toughness ($\text{K}{}_{\mathrm{Id}}$) theory, and (3) The dynamic analysis, propagating crack energy or toughness ($\text{R}{}_{\mathrm{ID}}$ or $\text{K}{}_{\mathrm{ID}}$) theory. These three concepts are examined in the light of measurements of unstable fracture and crack arrest in wedge-loaded DCB test pieces together with a fully dynamic analysis of the experiments.     

Mechanism of the effect of hydrogen on fatigue crack propagation in structural steels

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 39–42, May–June, 1988.     

氢渗透电流法监测海洋大气中钢材的腐蚀速率

用Devnathan-Stachurski双电解池技术,对16Mn钢表面干湿循环时氢渗透现象进行了研究,结果表明不同表面液膜下,都有氢渗透电流的存在.氢离子渗透量与试样腐蚀失重之间存在线性关系.利用此线性关系制作的实时监测氢渗透电流的传感器,用以记录实际海洋大气中氢渗透电流,并根据氢离子渗透量与腐蚀失重之间的线性关系对海洋大气中钢材的腐蚀速率进行预测.结果表明,实际海洋大气中,氢渗透电流与环境湿度存在着对应关系,环境湿度由大变小时,氢渗透电流由小变大.环境湿度交替变化,在试样表面完成干湿循环,促进了氢的渗透,实际海洋大气与摸拟海洋大气失重取得了较好的一致性.可以用氢渗透电流传感器实时监测海洋用钢在大气中的氢渗透情况及腐蚀失重情况.     

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

Abstract

The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10^-5 to 4·2×10^-2 s^-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twin boundary fracture on the fracture surfaces of solution annealed 304L and 304 steels, and grain boundary fracture on the sensitised 304 steel. It was found that from room temperature to the maximum HEE temperature, the HEE of the materials depended on the transformation of strain induced martensite and below the maximum HEE temperature it depended on the diffusion of hydrogen.     

Fatigue propagation of short and long cracks in gaseous hydrogen environment in 3.5NiCrMoV steel

Turbo generators for nuclear plants are mostly equipped with hydrogen cooling systems. Current practice of characterizing the growth of fatigue cracks on the basis of fracture mechanics primarily relies on fatigue tests for long cracks which are typically of several millimeters in length. However, in view of extended life for the plants, the damage tolerance evaluation of such fatigue-critical engineering components requires understanding of the propagation of cracks of significantly smaller dimensions. Then the near threshold of short cracks is investigated and compared to the behavior of long crack by experiments under 4 bar hydrogen atmosphere. The short crack fatigue propagation in hydrogen atmosphere is shown similar to that in air, growing faster than the long crack and at ΔK ranging below the long crack threshold; this effect is related to a reduced crack closure shielding. The propagation behavior of long crack under hydrogen atmosphere is shown similar to that obtained in air in the low rate range, i.e. when the maximum of the stress intensity factor K_max is lower than a critical level of about 16 MPa m^1/2 with higher crack growth rate than in high vacuum. This environment effect is related to the presence of residual water vapor in both gases. For higher K_max, much faster growth rates under hydrogen atmosphere in comparison to air and vacuum are observed and related to hydrogen assisted intergranular propagation combining fatigue and sustained loading damage. The results are discussed on the basis of micrographic observations supporting the involved mechanisms.