Heat Treat Tips: Induction Heating — Stuff You Should Know
During the day-to-day operation of heat treat departments, many habits are formed and procedures followed that sometimes are done simply because that’s the way they’ve always 
been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat Treat Today‘s 101 Heat Treat Tips, tips and tricks that come from some of the industry’s foremost experts, were initially published in the FNA 2018 Special Print Edition, as a way to make the benefits of that community available to as many people as possible. This special edition is available in a digital format here.
In today’s Technical Tuesday, we continue an intermittent series of posts drawn from the 101 tips. The category for this post is Induction Heating, and today’s tips–#29, #73, and #83–are from Dr. Valery Rudnev, FASM, Fellow of IFHTSE, “Professor Induction”, Director of Science & Technology at Inductoheat Inc., an Inductotherm Group company. Dr. Rudnev is a regular contributor to Heat Treat Today.
Heat Treat Tip #29
Induction Heating Non-Ferrous Metals & Alloys
Steel components by far represent the majority of hot worked and heat-treated parts for which electromagnetic induction is used as a source of heat generation. At the same time, many other non-ferrous metals and alloys are also inductively heated for a number of commercial applications. Induction heating of low electrically resistive metals such as Al, Mg, Cu, and others typically require using lower electrical frequencies compared to carbon steels, cast irons, or high resistive non-magnetic metals (such as Ti or W, for example) and metallic alloys. The lower value of electrical resistivity results in smaller current penetration depth (depth of heat source generation), making it possible to apply much lower frequencies without facing the danger of eddy current cancellation.
Heat Treat Tip #73
Induction Hardening Powder Metal
When induction hardening powder metallurgy (P/M) materials, it is good practice to have a minimum density of at least 7.0 g/cm3 (0.25 lb/in.3). This will help obtain consistent induction hardening results. When hardening surfaces that have cuts, shoulders, teeth, holes, splines, slots, sharp corners, and other geometrical discontinuities and stress risers, it is preferable to have a minimum density of 7.2 g/cm3 (0.26 lb/in.3). Low-density P/M parts are prone to cracking due to a penetration of the gases into the subsurface areas of the part through the interconnected pores. Interconnected pores contribute to decreased part strength and rigidity compared with wrought materials. In addition, the poor thermal conductivity of porous P/M parts encourages the development of localized hot spots and excessive thermal gradients and also requires the use of quenchants with intensified cooling rates to obtain the required hardness and case depths. This is so because an increase in pore fraction and a reduction in density negatively affect the hardenability of P/M materials compared to their wrought equivalents.
Heat Treat Tip #83
Induction Hardening Cast Iron
Induction hardening of cast irons has many similarities with hardening of steels; at the same time, there are specific features that should be addressed. Unlike steels, different types of cast irons may have similar chemical composition but substantially different response to induction hardening. In steels, the carbon content is fixed by chemistry and, upon austenitization, cannot exceed this fixed value. In contrast, in cast irons, there is a “reserve” of carbon in the primary (eutectic) graphite particles. The presence of those graphite particles and the ability of carbon to diffuse into the matrix at temperatures of austenite phase can potentially cause the process variability, because it may produce a localized deviation in an amount of carbon dissolved in the austenitic matrix. This could affect the obtained hardness level and pattern upon quenching. Thus, among other factors, the success in induction hardening of cast irons and its repeatability is greatly affected by a potential variation of matrix carbon content in terms of prior microstructure. If, for some reason, cast iron does not respond to induction hardening in an expected way, then one of the first steps in determining the root cause for such behavior is to make sure that the cast iron has not only the proper chemical composition but matrix as well.
If you have any questions, feel free to contact the expert who submitted the Tip or contact Heat Treat Today directly. If you have a heat treat tip that you’d like to share, please send to the editor, and we’ll put it in the queue for our next Heat Treat Tips issue.
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