Fig. 1. Typical graph for determining total case depth (courtesy of Surface Combustion, Inc.)

Question:
We have a project for deep-case carburizing of wind-energy components. Case depths vary between 0.080 inch and 0.225 inch. For the samples done so far, most of the actual case depths have been shallow. We are used to doing the more typical 0.015-0.060 inch case-depth ranges at temperatures of 1650°F with a carbon potential in the 0.85-0.90%C range. We need to get more accurate process times. Can you help?

Fig. 2. Carburizing simulation cycle prediction (courtesy of Aichelin USA)

Answer:
There are several ways in which to address the question of how to determine the carburizing time, temperature and atmosphere potential needed for deep-case carburizing.

First, it is important to understand if you are dealing with total case depth (TCD) or effective case depth (ECD). If effective case, know if it measured at 50 HRC (513HV) or 52.5 HRC (550 HV) – the latter being common in Europe and various other parts of the world.

The selection of carburizing temperature and surface carbon content is dependent on a number of factors, not the least of which is material and application end use. In the wind-energy market today, many companies processing 17CrNiMo6 and 4820 material are using preheats around 1475°F (800°C) and carburizing temperatures of up to 1740°F (950°C). The temperature is typically lowered to around 1550°F (845°C) and the parts stabilized at these temperatures prior to quenching. Carbon potentials (boost) are typically in the order of 1.0-1.1%C, while final carbon content is around 0.80%.

Approximate time to achieve a given case depth can be determined by looking at various charts/graphs (Fig. 1) or consulting various reference books (e.g., Practical Data for Metallurgists-The Timken Company), which determine total or effective (carburizing and diffusion) time as a function of case depth and temperature. Today, we can also determine case-depth requirements using one of the various carburizing simulators on the market being promoted by various OEMs and third-party companies such as Super Systems Inc. (CarbCalc II). These packages can even suggest carbon potential (e.g., oxygen probe) settings.

By way of example, Figure 2 is a simulation of a carburizing cycle that predicts a profile for an effective case depth of 0.095 inch (2.40 mm) at 0.45%C.