High temperature cracking of nickel chromium austenitic steels.

On the basis of published work and the experimental results of this investigation, a detailed classification of high temperature weld cracking is proposed. Two major types of cracking are recognised: Type 1 - separations along boundaries decorated with liquated second phase resulting from microsegregation, (solidification cracking in the weld metal and liquation cracking in the HAZ (Heat Affected Zone) ): Type 2 - separations associated with relatively clean grain boundaries caused by grain boundary sliding in the solid state and they are synonymous with creep rupture. This form of cracking is referred to as ductility dip cracking because it is believed to be associated with a ductility minimum which occurs in some alloys. In order to study high temperature weld cracking in detail, a reliable and reproducible means of producing cracks is required. To fulfil this need, an experimental horizontal tensile machine was designed and constructed. The high temperature weld cracking resistance is expressed in terms of the applied stress and overall extension necessary to initiate cracking. Using this apparatus, the effect of grain size on the HAZ (Heat Affected Zone) cracking of AISI 304 (an l8Cr 1ONi non-hardenable austenitic steel in which the grain boundaries are free to migrate) was investigated. A linear relationship between applied cracking stress and the reciprocal of the square root of the grain size was found. This is consistent with the cracking theories. Bead on plate tests carried out on a more complex alloy, A286 (a 25Ni 15Cr intermetallic strengthened austenitic steel in which the grain boundaries are not free to migrate) verified the adverse effect of a coarse grain size. Cracking occurred in the HAZ (Heat Affected Zone) without an applied stress being required, and this was due to the formation of titanium rich liquid phases which wet the grain boundaries and render the steel hot short. Detailed light and electron microscopy complemented by electron- micro- probe- analysis revealed the cracking mechanism to be two stage: Initiation by liquation (Type 1) and propagation by a solid state creep rupture process (Type 2). A 0.39 wt% boron addition to the basic composition of A286 was found to improve the cracking resistance by refining the parent metal grain size and making the steel more resistant to grain coarsening during welding. The boron was also found to alter the elemental solidification sequence of A286 and it avoids the formation of the white etching hot short regions.