Long before many of today’s technological advances, people have heat-treated metals to improve their physical and chemical properties for a given application. In the middle ages, blacksmiths forged and tempered metals (albeit in a relatively crude fashion) to create blades, tools, and goods for everyday life. Now, metallurgists and material engineers have a much broader array of specialized techniques and equipment to tailor materials to specific applications.
But there are many different heat treatments, such as quenching, tempering, aging, stress relieving, and case hardening. To eliminate confusion, here’s a look at the most common heat treatments, along with their purposes and their pros and cons.
This shaft is undergoing induction hardening. Localized heating on the shaft (red/white glow) is immediately followed with a sprayed-water quench that rapidly cools it.
Annealing involves elevating a metal’s temperature until it is in an equilibrium state, as defined by its phase diagram. It is used to change the metal’s physical properties such as its hardness, but there can also be local chemical changes, depending on phase transitions. Annealing treatments usually follow machining processing, such as machining and grinding, or even other heat treatments such as quenching.
Quench and tempering. Quenching involves heating steel above its critical temperature and holding it there long enough to let the microstructure fully change to an austenite phase. The steel is then quenched, a process that rapidly cools the steel by placing it in water, oil, or a polymer solution. This “freezes” its microstructure. What the steel is quenched in to cool controls the cooling rate, and the cooling rate determines the post-quench microstructure.
Metallurgists use the metal’s time-temperature-transformation diagram (TTT diagram) to predict the resulting microstructure, whether martensite, bainite, or pearlite. With these structures, ferrous alloys with a carbon composition greater than 0.3wt%. can be extremely hard (>60 HRC), especially the martensite structure. But the increased hardness comes with decreased toughness.
Tempering, an annealing process, follows quenching. Steel becomes extremely hard and brittle after quenching, so it undergoes another step to reduce its hardness and increase its ductility, all while maintaining its microstructure.
Metal parts are loaded into baskets, then pulled into the carburization furnace at Advanced Heat Treat Corp. There, they will be heated above the metal’s critical temperatures.
Tempering a steel below its critical temperature lets it retain its martensitic structure but, if tempered long enough, it gets converted to a mix of ferrite and small carbides, the exact size of which depends on the tempering temperature. This makes the steel softer and more ductile. The key tempering parameters are temperature and time, and they must be precisely controlled to create the desired final hardness. Lower temperatures maintain higher hardness while removing internal stresses, and higher temperatures reduce hardness.
After initial casting or machining, quenching and tempering gives the steel the hardness and strength for making parts with material characteristics. Parts can then be machined to a final state. Quenching and tempering distorts the metal, so parts always go through these two processes before final machining. For parts with additional heat treat processes used to modify surface properties, quench and temper determine a part’s core properties such as hardness, strength, and ductility. (Additional surface hardening treatments will be covered later.)
Stress relieving. Stress relieving, an annealing step, follows grinding, cold working, welding, or final machining, and is done after the metal has been quenched and tempered to a desired microstructure and strength. This means special consideration must be given to ensure the workpiece is not annealed too closely to its tempering temperatures. This prevents changing the previously achieved hardness and microstructure.
Stress relieving removes internal dislocations or defects, making the metal more dimensionally stable after final processing, such as gas or ion nitriding. Stress relieving is not intended to significantly change the metal’s physical properties; changes to hardness and strength are, in fact, unwanted.
Precipitation hardening. Precipitation hardening is a special annealing step also known as age hardening due to certain metals hardening over time at sub-critical temperatures. As noted, this method of strengthening metals is limited to those that have undergone quenching and are an over-saturated solution, meaning the material is in a non-equilibrium state with regard to the phases present.
In these alloys, the over-saturated martensite solution is heated (500° to 550°C) and held for 1 to 4 hours, letting precipitates uniformly nucleate and grow. This results in a non-distorted, high tensile and yield strength steel with better wear properties than in its unaged condition. The precipitate phases, composition, and sizes depend on the alloy being aged, but all have the same general effect of strengthening the material.
Not all ferrous alloys are eligible for this hardening mechanism, but martensitic stainless steels such as 17-4, 15-5, and 13-8 are excellent candidates, as well as maraging steels. (The term “maraging” combines the two words "martensitic" and "aging." Those steels have superior strength and toughness without losing malleability, but they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process.) In these alloys, the over-saturated martensite solution is heated (500° to 550°C) and held for 1 to 4 hours, letting precipitates uniformly nucleate and grow. This results in a non-distorted, high tensile and yield strength steel with better wear properties than in its unaged condition.
Induction hardening. Induction hardening is much like quenching, with one distinct difference: Heating in induction hardening is selective. That’s because in induction hardening, heating is carried done by via magnetic coils designed to match the part’s geometry. This means critical part features can be hardened while the part’s core is not. Instead, the core retains the metal’s strength and ductility. Just as in traditional quenching, it is done using water, oil, or a polymer solution.
Induction hardening can be done on steels with a carbon content greater than 0.3wt%, and to parts with sizes and geometries that can have induction coils designed for them. Induction hardening also significantly reduces processing times needed to harden parts and decreases the risk of decarburization. Unlike traditional heating and quenching, induction is a surface-limited heat treatment with hardened depths ranging from 0.5 to 10 mm.
Transmission hubs are gas nitrided in stacks, which lets the nitriding gases (ammonia) flow between parts to fully heat treat the surfaces. Parts are stacked as high as the vessel’s working volume to maximize the process’s efficiency.
Case hardening heat treatments, which includes nitriding, nitrocarburizing, carburizing, and carbonitriding, alter a part’s chemical composition—unlike previously mentioned annealing techniques—and focus on its surface properties. These processes create hardened surface layers range from 0.01 to 0.25 in. deep, depending on processing times and temperatures. Making the hardened layer thicker incurs higher costs due to additional processing times, but the part’s extended wear life can quickly justify additional processing costs. Material experts can apply these processes to provide the most cost-effective parts for specific applications.
Carburization and carbonitriding. Carburization is ideal for parts requiring extra hardening on the surface for wear resistance but need a softer core for superior strength. Carburization is a high temperature process (900 to 950°C) that involves the addition and diffusion of carbon into the steel. Those temperatures are above steel’s critical temperature, so subsequent quenching lets the carbon-rich surface form martensite while the core remains a softer ferrite and/or pearlite structure. Hardened depths can be as thick as 0.25 in., depending on the amount of time the part spends soaking at carburization temperatures.
As mentioned, the advantage of carburization is a deep wear resistance layer with high hardness. This is ideal for gears, blades, and cutting tools. Carburization creates hard, durable parts from lower cost alloyed steels and low carbon steels, such as 1008, 1018, and 8620. For alloys with higher carbon content (>0.3wt% carbon), carburization has minimal or even detrimental effects because the carbon in the original alloy could lead to a through-hardened, or bulk martensite structure. It should be noted also, that carburization temperatures cause some part distortion.
For lower carbon steels without significant amounts of alloying elements that promote hardening, adding nitrogen to the process can increase surface hardness. Adding nitrogen is called carbonitriding. Carbonitriding is commonly performed at slightly lower temperatures than carburizing (850°C), so distortion is less, but it also reduces hardening depths (for comparable processing time). The hardened surface created during carbonitriding, while thinner, does have greater hardness and resistance to elevated processing temperatures (such as tempering and stress relieving.)
Nitriding and nitrocarburizing. The alternative to the high temperature carburizing/carbonitriding is nitriding/nitrocarburizing. It also produces hardened surface layers and similar wear resistances, but it diffuses nitrogen throughout the surface layer (not carbon), and it uses sub-critical processing temperatures. Typical temperature ranges for nitriding range from 450° to 575°C. This means parts can be processed in their final machined state and undergo little to no distortion, so little post-nitriding machining is required (if any). The lower temperatures also maintain the desired core microstructure and physical properties while modifying the surface layer for the given application. One note to consider when selecting nitriding: Inform the heat treater as to any stress relief, aging, or tempering temperatures to prevent altering core properties.
Unlike carburization, which is limited to lower-carbon-content steels, a broad range of alloys can be given surface hardnesses of 600 to 1,200 Hv via nitriding. But alloys best suited for nitriding typically contain nominal amounts of the microalloying elements: Cr, V, Ti, Al, and Mo. Nitriding can be extremely beneficial for stainless and tool steels containing large amounts of chromium (10+wt%). These nitrided steels can have surface hardness well above 70 HRC equivalent, perfect for long-term wear resistance.
Nitriding is not limited to these types of ferrous alloys either, as low carbon steels can be hardened as well. In addition to creating a hardened, wear resistant surface, nitriding also forms a compound zone. Compound zones are nitrogen-rich layers formed on the surface during nitriding which are hard, wear-resistant (>60 HRC equivalent), and corrosion-resistant. This benefits low carbon and low alloyed steels which would not be considered for harsh environmental conditions if not for the presence of a compound zone.
Depth of hardening for nitrided/nitrocarburized alloys typically range from 0.005 to 0.030 in., depending on the process’s time and temperatures. Deeper hardened layers require more time. Compound zone thicknesses can be up to 0.002-in. thick, and it’s a function of which alloy is being nitrides, the time, and temperature. How the part is nitrided also affects zone depth. Nitriding can be performed via gas or ion (plasma).
Gas nitriding uses cracked ammonia as the nitrogen source and is done in a positive-pressure environment. It’s ideal for large quantity batch processing and is also excellent with regards to temperature uniformity and nitriding parts with deep holes or channels. Gas nitriding is not recommended for porous parts because gas flowing through pores can cause severe embrittlement.
Ion nitriding is excellent for selectively nitriding, since parts can be masked off from the plasma to prevent nitriding. Ion nitriding is performed by applying a potential electrical difference across an anode and the part (the cathode) in a vacuum. This potential difference forms a nitrogen plasma (a unique purple glow) which forces nitrogen atoms into the part’s exposed surfaces.
Plasma nitriding is well-suited to alloys, such as stainless steels, since it quickly breaks down passive oxide surfaces. Typically, ion nitrided steels have thinner compound zones than their gas nitrided counterparts due to the plasma’s constant sputtering. But this can be ideal for certain applications, such as gears, where contact stresses could harm surfaces with excessive compound zones.
A purple glow surrounds these parts being ion (plasma) nitrided. It is caused by ionized and excited nitrogen molecules and atoms bombarding the part surface due to the applied potential. Only surfaces exposed to the plasma are nitrided.
In comparing nitriding and nitrocarburizing, the latter is typically performed at higher temperatures (575°C) and a source of carbon is used. The addition of carbon forms a harder, more wear-resistant, and higher lubricious layer. Thicker compound zones can also be formed by nitrocarburization. For comparison, a pure nitrogen nitriding environment forms a hard and wear-resistant layer, but less so than nitrocarburization. So why not always introduce use nitrocarburization? Introducing carbon can increase the surface porosity, which is bad for parts with large contact stresses. The resulting layer is also less ductile.
Material selection also drives which processing techniques are best for an application.
This general guideline explains an array of heat treatments. But it is important for engineers to keep in mind the following questions about their part design when considering heat treatments: What forces are my parts subjected to? What environment are they working in? Does the application require distinct properties for the surface, core, or particular surface regions? The answer will guide the selection.
This was written by Rich Johnson (materials & process manager), Edward Rolinski (sr. scientist), and Mike Woods (president) at Advanced Heat Treat Corp. If you have any questions regarding heat treatments, please feel free to contact them at 319-232-5221.