Heat Treatment

What do you see in this photo? Do you only see a combine as harvesting food? Stop! Look and think deeper! Consider the thousands of parts and components used in harvesting equipment that only work because they are heat treated. Then look around in your world, because nearly everything you can see or touch is often connected with metals that have been heat treated.

Welcome to the Heat Treatment Master Control Manual. There are lots of books on heat treatment, but this is the first manual that I know of that gives you all the information needed to properly heat-treat a large percentage of the popular grades of tool steels, alloy steels, stainless steels, and other specialty metals produced under American standards. It is not a manual for metal grades produced in the European or Far Eastern grade designations. It is based on American grades of metals. Can some of the techniques be utilized with overseas grades? Yes, but not without careful study and proper metallurgical analysis. As you will see, thermal treatment is a very scientific as well as a very tightly controlled and specific treatment. It is not a hit-or-miss subject or a magician’s sleight of hand. It is proven and it works!

Welcome to the world of hot and cold thermal treatment! For centuries, people have called the process of hardening metals “heat-treating.” Heat-treating is, as the name implies, “treating metal with heat.” The heat can be applied to metals by using a furnace, an open flame, a forge, or an electrical induction coil. Furnaces are the most common method of heat-treating, and an endless range of furnace designs are available to do many more different types of heattreatment processes.

Heat treatment could be considered “old school” because for decades our limited thinking was that only heat could cause transformation in metal to create a better, finer grain structure. In fact, only a very few manufacturing companies understood grain structure and its real importance to the longevity of metal. If a company was large enough, they might have an on-site metallurgist who watched over the heattreating processes. But, likely because heat treaters can’t easily see grain structure, it was decided that hardness would be the controlling standard.

We can’t cover all safety issues and needs in this manual; we can only deal with some obvious types of potential problems. Be sure to discuss your safety with the safety administrator in your location. It is vitally important that proper safety measures be observed and applied before performing any heattreating or subzero-cooling processes. Heat-treating temperatures run as high as 2400 °F to as low as – 320 °F. No one should ever assume a part is safe to touch, even if it doesn’t show any visible heat or flesh-killing cold. It can take hours for parts to return to room temperature from either extreme.

Heat-treating is an exact science. It is not black magic nor is it theoretical in any shape or form. If a person grasps and puts into action the straightforward, simple scientific steps of controlling time, temperature, and transformation (TTT) on identical pieces of known metals in a properly calibrated furnace, the results of every load put in the furnace will come out exactly within the stated limits. Every single time.

Metals can be traced back in human history to the elements copper, silver, lead, and iron, which may have been found in the 4000 BC time period; no evidence supports a firm date, and some feel it may have been used prior to that. Gold was found, it is believed, roughly around 3000 BC. The proper term in metallurgical terms is smelted. Smelting encompasses melting, or other means such as fused by pounding metal components We know smelted iron was developed around 1500 BC. The early metals were not very sophisticated, but they could be cast, hammer pounded to eliminate some slag and dross, then formed by recasting or pounding, and finally polished to a luster finish. The Bible and historians wrote of how the metal was heated, the dross or slag was skimmed off the surface to purify it, and eventually poured into molds. Today (early 2014) there are 118 elements in the periodic table, 66 of those metals. Until the 1600s, only 12 elements were known to exist.

To understand the heat treatment process, it can help a great deal to understand the basic steel-making process and how the chemistry (addition of elements) works to help make useful tools or products.

Stainless steel was discovered when Harry Brearley was experimenting with alloying steel to prevent fouling in rifle barrels and to prevent corrosion. The alloy he was trying to introduce into the iron rifle barrel steel was chromium. When he realized he had steel that didn’t corrode (stain) as much, he named it “stainless steel.” He also introduced how to use his new 12.8 % chromium steel to make cutlery that resisted rusting.

Don’t panic or get concerned. We’re not going to delve into the full study of metallurgy. You do need to know a few basic pieces of information and a couple of terms so you can fully grasp the heat-treating process. The hope is you’ll read through these few pages and glean the reason and understanding for the actual heat-treat training coming later. This will be brief.

There are two basic types of furnace environments: open atmosphere and controlled atmosphere. Within these two basic atmosphere types, there are a multitude of furnace designs for all types of metal processing. We’ll look at both types because the atmosphere is extremely important to both the heattreated part and the engineer who designs the parts to be heat-treated.

This chapter describes some of the tools every furnace room should, or might, contain. Some of these items might be obvious, but it needs to be considered for operator safety and the safe processing of the parts.

We have already discussed when metal reaches roughly 900 °F, decarburization begins and accelerates as the temperature gets hotter. 900 °F is also when heat becomes visible and can be seen by the human eye in a pitch dark room. But, that is also when carbon in the outer surface can be lost into the atmosphere and without enough carbon there can be no hardening.

As mentioned earlier, grain structure is the most important result of heat-treating and hardening steel. Hardness is an important part of the equation, but all too often it becomes the prominent or only factor because examining the grain structure is beyond the capabilities of many heat treaters. Grain size and structure will also have an effect on the mechanical properties of any heat-treated metal.

There are a number of different thermal treatments that are applicable to hardening steels. Notice the chapter title doesn’t say “heat treatments” because as you gather by now, it isn’t heat alone that affects steel’s hardness, structure, usefulness, or longevity qualities. There is also not a common temperature, or a single timing recipe, that applies to all the grades of steel, because each grade of steel has its own unique blend of chemistry. The percentage of carbon to iron, as well as the alloy content and alloy volumes, changes the exact recipe for each grade of steel. The metal must go into solution (Ac3) for heat treatment to occur. Each of the elements have their own melt point that allows the metal to achieve that proper solution, where the elements blend together to make one strong cohesive alloy.

Annealing steel produces a refined grain structure that is typically meant to aid the machinability of the steel. There are occasions where medium alloy steels will actually machine better if they have some hardness in their properties. Annealing changes the strength and hardness of steel, improves ductility or softening, and relieves some stresses.

Loading the furnace sounds like a rather innocuous endeavor, doesn’t it? But it’s really a very important part of performing the heat-treating process.

SPECIAL NOTE: In the recipes shown in Chapter 30 we generally show the preheat temperature different than other publications. The reason for this variation is our concern in the grain structure. For decades the preheat temperature has been 1200 °F, and now without reasonable explanation, it has changed. For example: A2 is now listed by others to preheat at 1450 °F to 1500 °F and we recommend 1200 °F. We prefer the 1200 °F temperature because it stays under the Ac1 of 1421 °F where austenite starts to form. By exposing the parts to a longer, higher heat in the austenite stage, greater amounts of retained austenite may be found after the heat-treating is finished. Most of the firms that suggest the higher temperature preheat also do specify cryogenic processing that will transform the excess austenite into martensite, but in the real world, there are parts that will never be processed through cryogenics. In addition, it seems to this author that it’s poor practice to make a bad part that requires a corrective process, when we can make a good part and then make it better!

Not many years ago, before heat-treating furnaces were commonplace, heat-treating was often accomplished in open forges. It may have looked peculiar but the craftsman would take a long steel rod with a magnet hanging from the end of the rod by a wire. He would lower the magnet unto the red hot steel. Once the magnet was no longer attracted to the steel, the craftsman knew the steel had reached its upper critical temperature. He would look at his pocket watch and start the timing of the soak. He understood that when the steel went in-solution, even though the part still held its physical shape, the carbon matrix and the alloy content were dissolving into a full solution, almost as a liquid. Thus, he knew because there was no longer any magnetism present. As the chemistry in steel became more complex, heat-treating furnaces and controllers were improved to give us more accurate control. In today's world we're fortunate to have very accurate heattreating furnaces with good temperature controllers and pyrometers to track and control the temperature of the steel within a few degrees.

This is the third and last T in the TTT diagram and it the final, Transformation phase of an austenite structure, ‘transforming’ the steel into a fine grained, hardened martensite structure. There is some additional transformation that occurs during the tempering step, and though it is minor, it is still important in the final grain structure. It occurs because the tempering temperature will reenter the martensitic transformation temperature zone.

Tempering is extremely important, but it also seems like the least understood part of heat-treating.

Cryogenics is the continuation of heat treatment and is the last step that completes the thermal treatment processing of metals. After ferrous steel part/s are heat-treated and tempered, the part’s grain structure can be improved by subjecting the part to a deep cryogenic temperature (–320 °F) soak. The process involves taking the parts down from room temperature to –70 °F to –100 °F using dry nitrogen gas.

The following 3 chapters (Chapters 22, 23, and 24) of this manual deal with the hands-on, step-bystep heat-treatment of air-, oil-, and water-hardening steels. Each of the chapters is complete from start to finish for each of the three distinct grades. By reading one grade, you get a complete picture of that steel with heat-treat requirements and thermal processing.

During any project there can be a whole batch of parts or components that must first be engineered, then rough machined. This amounts to a good deal of time and money. The parts must be made slightly oversize to allow for finish grinding. We’re going to pick up on the process right at this point and walk through the heat treat process. Keep in mind it can take a few hours, or even months of design, calculations, and tedious research to put together the design and machine the parts to be ready for heat-treating. All the hours of designing, prepping the pieces of steel, machining to exact tolerances, all boil down for either failure, limited success, or knock it out of the ball park greatness, in just a couple hours of heat treatment. All the time and money put into the project now rest in the hands of the heat treater and he can affect the outcome of the project for success, or failure. To make sure the parts will give long life and be properly heat-treated, we will walk you through the process step-by-step.

During any project there can be a whole batch of parts or components that must first be engineered, then rough machined. This amounts to a good deal of time and money. The parts must be made slightly oversize to allow for finish grinding. We’re going to pick up on the process right at this point and walk through the heat-treat process of oil-hardening. Keep in mind it can take a few hours, or even months of design, calculations, and tedious research to put together the design and machine the parts to be ready for heat-treating. All the hours of designing, prepping the pieces of steel, machining to exact tolerances, all boil down for either failure, limited success, or knock it out of the ball park greatness, in just a couple hours of heat-treatment. All the time and money put into the project now rest in the hands of the heat-treater and he can affect the outcome of the project for success or failure. To make sure the parts will give long life and be properly heat treated, we will walk you through the process step-by-step.

Water-hardening steels are direct-case hardening steels. Their carbon content is high enough to allow a good case-hardening to be accomplished without carburizing. W1 is the tool steel designation for C1095 carbon steel, but is made under very controlled conditions. The scrap used for W1 is tightly controlled for chemical purity to assure a clean, certifiable, internal structure. C1095 does not have that level of purity and only reports the basic chemistry and not the trace elements.

As you’ve gathered by now, if there is no carbon, or there are very low levels of carbon, then virtually no appreciable heat-treatment hardness will be realized. But, if we add carbon to the outer surface of the parts, we can create a case hardness over the core: the core being the inner structure of low carbon material that cannot be heat treated. This process is known as carburization. There are several ways to add this carbon layer to parts that we will look at here, but there are also other ways to treat lower carbon steels and even nonferrous metals, to put a case hardness on them.

When any tool or part fails from breakage, wear, corrosion, stress, or for any other reason, there is usually a cause. It is safe to assume that any tool will eventually wear out; but what really needs to be considered is, “Is the tool wearing out prematurely?” That’s the main issue and focus in this chapter. We are going to cover a lot of areas in this chapter, so we will use a good many subtitles. If you are going to evaluate a failure, make sure you do not assume anything, and make sure you do not reach a conclusion until you investigate all the evidence. Investigating a failure has a good deal of similarity to a crime scene investigation. It takes a lot of digging to get all the potential reasons for a failure, and it may very likely require a full metallurgical analysis. It is important to consider doing the analysis yourself, and leave no stone unturned. Remember that a metallurgist may typically zero in, and look for a problem in the internal structure first. Also remember that the problem may be in an area that cannot be seen.

Now, this is an interesting subject! I have been employed by two vacuum furnace manufacturers and a large induction furnace manufacturer; I have also represented two gas and one electric box furnace company and owned a cryogenic processing business. This culminates with 35 years of tool steel and alloy steel heat-treating experience. Despite all of my experience, I’ve never seen a definitive, straight forward directive on this subject.

Tool steel or medium carbon alloy steel selection isn’t difficult if you simply study the application to determine what is being requested, what forces are involved, and make your best effort choosing the best steel for the application is!

WARNING: Sparks coming off a grinding wheel are 2800 °F, or higher. Safety must be the foremost thought when doing these, or any tests. Full face masks, all body parts and hair covered, heat resistant gloves, no combustibles near by, no loose clothing.

The following grade charts give you detailed heat & thermal treatment information needed to process most of the available metals commonly used in today’s manufacturing world. This gives you a ready reference in one location for these metal treatments. They are listed alphabetically and numerically.