August 2020 Volume 2

MATERIALS

Figure 1: Table 7 from ASTM Standard A22 – Revision 10 showing the relationship between carbon content and expected hardness values for 100% and 50% martensitic transformation

Also notable in Figure 1 are the expected hardness values for a 50% transformation to martensite, which are notably lower. Suppose a 1045 forging is checked for as-quenched hardness and a value of 45 HRC is obtained. Under these circumstances, it can be reasonably inferred that the quench rate was not sufficient to achieve the necessary martensitic transformation—assuming the part was held at the austenitizing temperature long enough. The real danger of spotty hardening is that certain areas of the forging can be considerably harder than the one location that was checked. If the tempering protocol is based on a low as-quenched hardness, the part may end up within spec for final hardness at the location inspected, but it could still be considerably harder at other locations. This can lead to massive tooling failure in machining or brittle failure in service. So how can a heat-treat process ensure that spotty hardening does not happen? First, consider how parts are arrayed or fixtured in relation to the flow of the quench media. It must be ensured that parts are adequately spaced so that quench liquid is not impeded between parts. Whenever possible, parts with large, flat areas should be fixtured vertically so that fluid moves past the flat surfaces and is not blocked by them. Under no circumstances should parts be heaped in a basket. Next, the selected quench media and quality of the quench tank is critical. Water is often selected as a quench media for low hardenability alloys, however, the velocity of the fluid in the tank should be understood, and it must be ensured that the temperature of the water is controlled at levels under (at least) 80-90°F, as the cooling performance of water degrades rapidly past this point. The starting temperature of the quench tank should be consistently

controlled within at least a 10°F window for each quench. This often requires a refrigerant chiller system to keep up with production demands. Quench tanks should be continuously or periodically filtered and/or cleaned out to ensure scale buildup in the tank does not degrade fluid velocity. Finally, the surface condition of the forgings is a critical factor. Heavy scale acts as an insulation blanket and prevents quenchmedia from removing heat at the necessary rate for low alloy forgings. Clean surfaces allow for quick and uniform surface wetting of the parts and optimal heat transfer. To ensure this is happening, parts should be cleaned prior to heat treating and should be processed in a controlled or nitrogen atmosphere furnace during heating. From the standpoint of forging design, care must be taken when using low-hardenability grades. These forgings should be uniform in cross section to help avoid spotty hardening. They should also be used for thinner cross-section parts – under two inches – as thicker cross sections can become very difficult to quench sufficiently to 90% or higher martensite. ■

Chuck Hartwig is the Director of Operations for Carburizing and Batch Hardening at ThermTech in Waukesha, WI. He holds a B.S. in Metallurgical Engineering from Colorado School of Mines.

FIA MAGAZINE | AUGUST 2020 41