Cold Cracks in Fillet Weldments of 600 MPa Tensile Strength Low Carbon Steel and Microstructural Effects on Hydrogen Embrittlement Sensitivity and Hydrogen Diffusion

Hydrogen flowing into a weld causes hydrogen embrittlement (HE), so to prevent cold cracks in the weld, HE must be suppressed. In this study, single-pass fillet weldments were fabricated using two different welding fillers with the same strength level as the American Welding Society standard. The specimen F-HNi, which had a high, reversible hydrogen content, exhibited cold cracking in the fillet weld. The HE sensitivity index (HE index) was calculated using the in-situ slow strain rate test (SSRT). The reversible hydrogen trap concentration (<i>Crev</i>) and hydrogen diffusion coefficient (<i>Deff</i>) were calculated using the permeation test. The formation of low-temperature transformation phases such as bainite and martensite increased the <i>Crev</i> and decreased the <i>Deff</i>, thereby increasing the HE index. In addition, it was determined that reversible hydrogen was most effectively trapped in the low-temperature transformation phase, as confirmed by silver decoration. We concluded that the cold cracks in the F-HNi specimen were associated with the large <i>Crev</i>, small <i>Deff</i>, and large HE index. HE sensitivity can be controlled by optimizing the microstructure, even when welding fillers with the same level of strength are used. To reduce HE sensitivity, it is important to reduce the number of reversible hydrogen trap sites, by reducing the fraction of the low-temperature transformation phase where the reversible hydrogen trapping most occurs.

Investigation of Hydrogen Diffusion Characteristics of the Heat Affected Zone of 2.25Cr-1Mo-0.25V Steel by an Electrochemical Permeation Method

The heat affected zone (HAZ) of 2.25Cr-1Mo-0.25V welded joint is a critical part for the safety of hydrogenation reactors. Hydrogen has a significant effect on the HAZ, studying hydrogen diffusion characteristics, such as: hydrogen flux and the effective hydrogen diffusivity has a remarkable value in investigating the hydrogen-induced material degradation mechanisms. In this work, an electrochemical permeation method was applied to study the hydrogen diffusion characteristics of HAZ. Then, the metallographic microscope and a software “Image J” were used to analyze the density of grain boundaries of HAZ. The effect of the post–weld heat treatment (PWHT, i.e. annealing) on the hydrogen diffusion characteristics of HAZ was also been investigated. The results show that after PWHT, the effective hydrogen diffusivity of HAZ increases from 1.63 × 10−7cm2·s−1 to 3.68 × 10−7cm2·s−1, the hydrogen concentration decreases from 1.92 × 10−4mol·cm−3 to 1.09 × 10−4mol·cm−3, and the hydrogen trap density decreases from 3.00 × 1026m−3 to 0.76 × 1026m−3. Thus, PWHT can significantly reduce density of grain boundaries, thereby reducing the hydrogen trap density, enhancing the hydrogen diffusivity and reducing the hydrogen concentration.

The Fatigue Threshold Computation of Steel in Hydrogen Environment by Shakedown Analysis

In this paper, the fatigue threshold of steel servicing in hydrogen environment is studied by means of shakedown analysis. First of all, the application of shakedown analysis on the fatigue threshold prediction is reviewed briefly. Secondly, the classical static shakedown theorem for elasto-perfectly plastic structure is modified to take hydrogen’s effect into consideration. In the proposed method, the effect of hydrogen is described by a yield stress associated with hydrogen concentration and hydrogen content in metal is assumed to be composed of the hydrogen in lattice sites and in reversible trap sites. The effect of plastic flow on hydrogen trap density is involved in by considering the plastic strain as the function of residual stress, which is expected to result in a lower bound of the shakedown load. Finally, the fatigue threshold of a steel is computed as example. The performance of the proposed method is discussed and compared with the results from references. The results indicate that the proposed method is of validity.