“Martensite is our friend,” so sayeth the heat treater, but what is martensite, really? And why is a tempered martensitic structure the single-minded goal of every heat treater when hardening steel? Let’s learn more.

In order to form martensite we need to heat steel into the austenite field (above Ac3) and quench rapidly enough from the austenite phase to avoid pearlite formation. The rate must be fast enough to avoid the nose of the Time-Temperature-Transformation (TTT) curve – the so-called critical cooling rate for the given steel. The formation of martensite involves the structural rearrangement (by shear displacement) of the atoms from face-centered cubic (FCC) austenite into a body-centered tetragonal (BCT) martensitic structure. By Dan Herring

Everyone knows what a flowmeter is, and, yet, few of us really understand them the way we should. The sad reality is that once flowmeters are installed and operating we tend to take them for granted. This can often lead to serious flow errors and potential process or safety issues that compound themselves over time. Let’s learn more.

A flowmeter is a device used for measuring the flow of gases or liquids. There are actually two different ways to measure flow – by volumetric means (Fig. 2a) and by mass-flow techniques (Fig. 2b). As heat treaters, we are probably more familiar with the volumetric-flow measurement of gases. The principle involves the displacement of the gas volume over time. Atmosphere furnaces, gas generators and combustion systems typically use these types of devices. Mass flow involves measuring the weight of a gas, and these are commonly found on vacuum furnaces that meter in gases for partial-pressure operation. By Dan Herring

Stress relief is a heat-treatment process that relies on slow cooling to achieve its desired effect, and it is influenced by a number of factors including the internal stress induced into the parts from the various manufacturing methods (e.g., bending, shearing, forging, sawing, machining, grinding, milling, turning, welding, etc.) and prior processing. The application end use ultimately defines the allowable stress state. So how does one perform a stress-relief operation? Let’s learn more.

Processes that depend on slow cooling (e.g., annealing, normalizing, stress relief) do so for a number of reasons – to relieve stresses, improve chemical homogeneity, soften a material for subsequent operations (e.g., machining), refine grain size and for such reasons as embrittlement relief or magnetic properties.[1] As a general rule, the larger or more complex the part, the greater the amount of internal stress present. By Dan Herring

Long referenced in military heat-treat specifications for pyrometry, AMS 2750 is now being used by NADCAP to audit pyrometric practices for special processes in the aerospace industry. Recent specification changes better define the requirements. Our focus will be limited to temperature uniformity surveys.

On a recent Internet search for the definition of pyrometry, one of the most interesting definitions I found was, “the art of measuring degrees of heat, or the expansion of bodies by heat.” It was the “art” in the definition I found interesting. by Jim Oakes

In his 100th Heat Treat Doctor column, Dan Herring stated that heat treating can best be defined as “the controlled application of time, temperature and atmosphere to produce a predictable change in the internal structure (i.e. the microstructure) of a material.” He went on to add that “metallurgists are responsible to predict the microstructural changes that will occur in a component.”

This article describes how computational thermodynamics and kinetics can assist the metallurgist in making such predictions. by Paul Mason

This is the third and four part of a four-part article on the principles of Gas Nitriding (PART 1-2). Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating (Ac₁) as defined by the Iron-Carbon Phase Diagram.

Gas Nitriding Reactions: Gas nitriding is typically done using ammonia with or without dilution of the atmosphere with dissociated ammonia or nitrogen (or nitrogen/hydrogen) in the temperature range of 925-1050°F (500-565°C). Ammonia (NH3) is allowed to flow over the parts to be hardened. By Dan Herring

Energy conservation has become one of the primary issues for the heat-treating industry in the new millennium. It is important in this new business climate to look at all opportunities for increased furnace efficiency.

In this article we will look at improving the weight ratio of part loading to cast or fabricated alloy tooling and improving the efficiency of other internal furnace parts. By Richard J. Grimm