Alloys and Acid Performance

When using laser or plasma processing on precipitation hardenable materials, the heat from the process is sufficient to melt a spot on the surface. At this point the pressurized gas is turned on. This forces the liquid metal off the piece, thus making the cut. Some of the liquid metal does not get forced away and instead resolidifies. In martensitic and martensitic precipitation hardenable stainless steels, this small region cools fast enough to form an untempered martensitic zone. In the worst-case scenario, this zone can form cracks from internal stresses. Any attempt at fabrication can lead to propagation of the crack into the base metal. When customers insist on plasma or laser cutting, extra material allowances are added in order to compensate for the potential for cracks. The customer should be advised that the plasma or laser cutting edges need to be ground smooth to remove any potential for cracking. This extra allowance sometimes is not sufficient: cracks sometimes propagate past the extra material. One potential solution to this cracking issue is to age harden the material from a softer condition immediately or soon after cutting, perform all fabrication and then reheat treat the entire fabrication. A better solution is to instead water jet cut since this technique does NOT form any liquid phase.

In the case of processing duplex via the laser or plasma, the chemical inhomogeneity of duplex materials will need to be addressed. With laser processing you can remove some of the Chromium, while oxidizing away the Molybdenum and Nitrogen found in the material. This results in lowering the P.R.E.N. to the point that it will not have the resistance of the base metal. Duplex is also subject to rust or pitting, as well as contamination due to chemical variations leading to more segregation within the microstructure. If these materials are kept at high temperature (°F) for an extended period, depending on the transformation kinetics you could end up developing sigma phase within an hour of reaching the desired temperature.