Valve Magazine - Winter 2009

Note the severe intergranular corrosion that occurred in just three weeks of service in nitric acid.

Figure 1

test reports say so, or have you confirmed the test reports are accurate and represent the actual casting that will be used?

Heat treatment

Recommended heat treatments may provide guidance for general corrosion

Figure 2

resistance or certain

mechanical properties, but they do not guarantee the highest quality. For example, proper solution annealing of

Figure 3

cast austenitic stain-
less steel requires
the correct chemistry holding at suffi-
cient temperature and time followed by a
rapid quench. If done properly, the result-
ing microstructure would appear as
shown in Figure 1, contrasting with Fig-
ures 2 and 3, which do not exhibit the
appropriate microstructure.

How often do you verify the quality of your cast austenitic stainless steel? Figure 4 shows a CF8M valve body that was not quenched at an acceptable rate.

Repair welding: competence and procedure It is impractical to expect 100% perfect castings every time; defects are inevitable. Repair of defects at foundries is commonplace, but it is often overlooked. I strongly recommend that you visit the weld repair room whenever visiting a foundry. Are the welders routinely qualified to any standard? Do they have weld procedures to control their processes? Do they follow those procedures? Is the equipment in good shape and appropriate for the types of repair being done? Are the filler metals the correct ones and are they stored properly? Are defects fully excavated before welding? This plenitude of questions, when left unan-swered, puts you at high risk for low quality. See Figure 5.

Stainless-steel passivation There are many ways for free iron to embed into the surface of stainless steel. In sand castings, reuse of sand previously used for carbon steel, cleaning of stainless castings with carbon steel shot or carbon steel wire brushes are examples. If no steps are taken to check for and remove the free iron, your stainless steel casting can rust on the exterior. Passivation is one way to remove this free iron. See Figure 6.