High Crack Sensitivity Research Articles

Efficiency of lightweight aggregates for internal curing of high strength concrete to eliminate autogenous shrinkage

High Strength Concretes (HSC) with extremely low water to binder (w/b) ratios are characterized by high cracking sensitivity which is a consequence of increased autogenous shrinkage. The major reason for autogenous shrinkage—self-desiccation—cannot be eliminated by traditional curing methods. The application of the concept of internal curing by means of saturated lightweight aggregate was applied and shown to be effective in eliminating autogenous shrinkage. The present paper describes an approach to optimize the size and porosity of the light-weight aggregate to obtain effective internal curing with a minimum content of such aggregate.

Compression Test to Reveal Surface Crack Sensitivity between 700 and 1100.DEG.C. of Nb-bearing and High Ni Continuous Casting Slabs.

In this study, the surface crack sensitivities in samples of a Nb-bearing steel and a high Ni grade steel from continuous casting slabs have been examined by simple compression tests at various temperatures between 700-1 100°C using a metallography method to evaluate the severity of surface cracks, and corresponding microstructure in the two grades of steel has been investigated. The results show that most specimen have cracks on the hoop surface after 50% compression in height in the temperature range of interest, and the critical hoop strain obtained from the metallography examination gives a clear cracking tendency for the two grades of steel. Microstructure observation revealed that the static precipitation of TNb(CN) before deformation and thin ferrite film along grain boundaries are important for controlling surface crack sensitivity for the Nb-bearing steel, while coarse grains with flat boundaries, and grain-boundary precipitation of Cu 2 S and flake-like T(CN) at lower temperature, is responsible for the high crack sensitivity in high Ni steel between 700-1 100°C, which implies Cu, S, Ti and N content should be kept as low as possible in this grade of steel, and surface temperatures of continuous casting slabs at the straightening point should be above 970°C for Nb-bearing steel while 980°C for high Ni steel to avoid transverse cracking.

Improved microstructure and properties of 6061 aluminum alloy weldments using a double-sided arc welding process

Due to its popularity and high crack sensitivity, 6061 aluminum alloy was selected as a test material for the newly developed double-sided arc welding (DSAW) process. The microstructure, crack sensitivity, and porosity of DSAW weldments, were studied systematically. The percentage of fine equiaxed grains in the fully penetrated welds is greatly increased. Residual stresses are reduced. Porosity in the welds is reduced and individual pores are smaller. It was also found that the shape and size of porosity is related to solidification substructure. In particular, a weld metal zone with equiaxed grains tends to form small and dispersed porosity, whereas elongated porosity tends to occur in columnar grains.

Influence of boron content on hot cracking sensitivity of weld heat affected zone in incoloy 800.

The influence of B content on the hot cracking sensitivity of the heat affected zone (HAZ) in Incoloy 800 was examined by Varestraint test and by hot ductility test using the thermal cycle simulator. The results indicated that the HAZ cracking sensitivity increased with the increase of B content until the B content exceeded about 0.01%, which is estimated to be the solubility limit, but the cracking sensitivity was decreased above 0.01% B. Similar relationship has been observed in the case of weld metal. However, the mechanism of the HAZ cracking seems to be somewhat different from that of weld metal cracking which depends on the extent of healing phenomenon, as the change of liquid film composition during the weld thermal cycle was observed. On the heating of weld thermal cycle, the composition of the liquid film in the HAZ was assumed to have been mainly that of Cr2B eutectic. However, the diffusion of B atom from grain into the grain boundary, in the weld thermal cycle after the initiation of grain boundary liquation, was so little at low B content below the solubility limit that (Fe, Ni)2B eutectic with a lower melting point than Cr2B eutectic became to dominate the composition of the liquid film, resulting in the extension of the brittle temperature range and the higher cracking sensitivity. On the contrary, at the B content higher than the solubility limit, the diffusion of B atom into the grain boundary was so sufficient that the liquid film preserved the composition of Cr2B eutectic which reduced such the brittle temperature range as described at low B content.The relation between hot cracking sensitivity of HAZ and B content of Incoloy 800 as mentioned above seems to be derived from the change of brittle temperature range due to such a difference of the liquid film composition.

Continuous Casting of High-Alloy Steels

A continuous casting process for high-alloy steels such as 13% Mn steel, 9% Ni steel, and stainless steels (AISI 631, AISI 310S) has been developed at Chiba Works of Kawasaki Steel Corp. Tensile testing at elevated temperatures and microscopic examination of the fractured surface of the corresponding steel specimen have shown that the high crack sensitivity of these grades is due to segregation and/or precipitation of sulfur and phosphorus at γ-grain boundaries, and for 9% Ni steel, additional large thermal expansion at γ/α transformation.

Maraging steels with high strength and plasticity

Steel 1, with σb = 260−280 kg/mm2, retains high local plasticity even in notched samples, but has a high crack sensitivity. Steel 2, with σb = 235−245 kg/mm2, has a relatively low crack sensitivity under static load. Additional studies of the structural strength are needed to determine the practical applications of steels 2 and 3.