Global markets have continued to put pressure on the price of diecastings, requiring manufacturers to find new opportunities to manage costs. Excluding labor, casting dies are one of the largest production expenses for automotive suppliers, and a very critical business item to ensure part quality and on-time delivery. If a die fails before its life has been reached, production cost will increase and the potential for los of business suddenly rises, if quality or delivery has been impacted. The single most important factor in determining the performance of die steel is the heat treatment process. To ensure the dies achieve their desired performance, confirming proper heat treatment of the die steel is essential. This guide will help casting personnel understand how to interpret a furnace chart and how this data can be used to judge if a proper heat treatment has been performed.
The first step to guaranteeing a successful heat treatment is to discuss with the heat treater the type of material being processed, the steelmaker’s recommended heat treating guidelines (see Figure 1), and the desired material properties.
It is highly recommended to review the North American Die Casting Association’s (NADCA) “Special Quality Die Steel & Heat Treatment Acceptance Criteria for Casting Dies.” This excellent publication provides a very detailed explanation for H-13 heat treatment quality requirements.
Heat treating H-13 die steel is divided into four major steps: preheating, austenitizing, quenching and tempering. Each step has a specific function with unique thermal requirements to optimize the steel’s mechanical properties.
To understand if the steel’s thermal requirements have been satisfied, a furnace chart needs to be provided to demonstrate that all heat treat criteria have been met. There are three distinct areas that must be verified via the furnace chart.
1. Pre-heating time and temperatures
2. Austenitizing time and temperature
3. Quench rate
Preheat time and temperatures. During the workpiece’s heat up, the core temperature (Tc) should not exceed 400°F/hour. The workpiece should be held at the first preheat temperature until the difference between the surface temperature (Ts) and core temperature (Tc) is less than 200°F. (See Figure 2.)
Once this occurs the furnace can be increased to the second preheat temperature and held until the difference between the surface temperature (Ts) and core temperature (Tc) is less than 25°F. (see Figure 3.)
Austenitizing time and temperatures. The workpiece should be quickly heated to desired austentizing temperature and held until the difference between the surface temperature and core temperature is less than 25°F.
Once this condition has been satisfied the part should be held at temperature for 30 minutes. (See Figure 4.)
Quenching rate. The workpiece temperature (Tc) should be dropped rapidly to 300°F with a minimum quench rate of 50°F/min between the austenitizing surface temperature (Ts) and 1,000°F. Typically, this will be done in less than 18 minutes. (See Figure 5.)
Tempering. After it has cooled (Ts) to <= 120°F, the workpiece must be loaded immediately for the first tempering cycle.
Heat treatment is the single most important factor in determining performance of the steel. The challenge for the customer is determining whether a proper heat treatment has been performed. By reviewing and understanding the key criteria of a furnace chart, the customer can have confidence in the steel’s delivered performance.
Mark L. Purtee is a Senior Staff Engineer at Honda of America Mfg. Inc. He has 29 years experience in aluminum casting, with responsibility for casting quality and material-related issues. Contact him at [email protected]