Long ago, the Egyptians, Greeks, and Romans developed basic techniques for heating and cooling metals to produce tools and weapons. They made an art of the heat treatment process, without all the electronic aid available today. They were, of course, limited in both what they could make and what they could do with it, but their development of the art allowed with the fabrication of more complex or harder objects.
Heat treatment today has become more standardized, to the point where a part of a certain size will be heated to a certain temperature, and held at that temperature for a specified time, before being cooled in either water, oil, air, or the furnace itself. The aim of a heat treatment process is to obtain a combination of mechanical properties that will meet a designated specification, or to meet properties specified by the customer. And coincident with this, to improve machinability.
There are major changes in metals and alloys when they are heat treated. There again, it depends on the type of heat treatment. It may be annealing, hardening and tempering, even aging, and largely depends upon the alloy that’s being heat treated.
At All Metals & Forge Group we are up to date on heat treatment procedures on a wide range of alloys, including carbon steels, alloy steels, stainless steels, tool steels, aluminum and its alloys, titanium and its alloys, nickel and its alloys, and super alloys.
The most important group of alloys, by any standard, is steel; carbon steels, alloy steels, stainless steels, stretching to tool steels. So, we’ll start by asking the question “Why do we heat treat steel forgings?” There are several reasons, and one of the most important is to improve machinability, where production of a finished machine component from a forging with rough dimensions is required. As such both machine tool life and finish machining productivity will be improved.
Then there are mechanical properties to meet a specification, or properties specified by the end user. In the latter case, it is left to the heat treater to choose the most effective and economical heat treatment times and temperatures. There are heat treaters who “know” their furnaces, who know that it’s difficult to follow a procedure that will give consistent results. It may be that the furnace itself has its own little idiosyncrasies, hence the need for the heat treater who knows it. On the other hand, the purchase order may specify the heat treatment cycle to be followed. But the person who “knows” the furnace is likely to step in to lend their knowledge that will guarantee the required results.
Although most steel forgings are subjected to some form of heat treatment, many grades of low-carbon steel, from say 0.10 to 0.25% carbon, are used in the forged condition. This is because the as-forged machinability is good, hence no heat treatment is necessary. The steel composition and the machining operations to be performed will determine the choice of heat treatment to be carried out. It may be a full anneal, a spheroidize anneal, a subcritical anneal, a normalize, or a normalize and temper.
The last two treatments mentioned, normalize and normalize plus temper, may produce the required strength and hardness in a steel, but for most steels a hardening, followed by a quench in oil or water, depending on hardenability, is used, followed by tempering. This will ensure the correct hardness, strength, ductility and impact properties. In certain cases, depending upon steel composition and properties required, either throughout the section being treated, or on its surface, flame or induction hardening, carburizing, nitriding or carbonitriding may be used.
There are certain principles that are primarily applicable to carbon and alloy steel forgings that also apply to martensitic stainless steels and certain other iron-base high-temperature alloys. We may consider heat treatment processes as involving four fundamental processes, namely heating, cooling, time at one or more temperatures, and the rate at which temperature changes occur in heating or cooling a particular part. It is important, for example, that heating rates be carefully considered in highly alloyed materials and in parts of complex shape. Cooling rates are important in producing the required microstructural (phase) changes, which in turn develop the required hardness and other mechanical properties.
The information here about the heat treatment of forgings is for general purposes and not the specific production or heat treatment of a forged part. For more information about the heat treatment of forgings, please contact a skilled metallurgist at All Metals & Forge Group.
