Training Manual BASIC HEAT TREATING
What is Heat Treating? -1- BASIC HEAT TREATING Heat treating is a process involving controlled heating and cooling of a solid metal to produce a desired change in the structure or properties of the metal. The ability to change a metal s properties (like hardness, strength and ductility) is vital in both the manufacturing process and end use of products made from metal. In general, all metals will respond in some way to heat treatment. Through heat treatment, they can usually be softened so as to be easily workable or hardened to be strong in service. However, depending on the kind of metal being heat treated, the type of response will vary substantially. Some common reasons for heat treating a metal include the following: Remove stresses in the metal that have been caused by forming or cold-working. Soften the metal so it can be more easily formed, cut or machined. Harden the metal to make it more wear resistant or to improve its cutting properties. Increase the toughness, so that the metal is both strong and flexible. Change the electrical or magnetic properties of the metal. Other heat treating processes, such as carburizing and nitriding, are used to change only the surface properties of the metal. How does Heat Treating Work? In very simplistic terms, most metals and alloys are manufactured by first melting a controlled mixture of elements (such as iron with small amounts of carbon, nickel, chromium, etc. for steel). As the molten mixture cools, it begins to solidify by forming tiny crystals. As solidification continues, these crystals keep growing until they arrange themselves in a regular, precise formation called the space lattice. The shape of the space lattice can take different forms depending on the type of elements used in the mixture. This lattice shape is what determines the properties of the metal. Some lattice shapes will make the metal very hard and brittle. Other lattice shapes will make it soft and ductile. Controlled heating and cooling (heat treatment) of the metal is used to change the shape of the lattice and thus the properties of the metal. Figure 1: Solidification of a metal from a molten liquid occurs as crystals form and grow in specific orientations to form a space lattice. Heat treating alters the shape of the space lattice to change the properties of the metal.
Why Heat Treat in a Vacuum? -2- The air we breathe contains a number of elements that can react with metals under the proper conditions. Moisture, oxygen, carbon dioxide and hydrogen are present in significant amounts in our atmosphere. Each can react to varying degrees with many different metals. While many of these reactions occur to only a small extent at room temperature, they are often greatly accelerated in the presence of heat. Consider the example of a piece of polished metal held over a heat source. It will eventually turn blue or black as the elements in the atmosphere react with the hot metal. In most cases, the heat treater tries to minimize the extent of these reactions during heat treating. The reactions cause changes in the surface properties of the metal that may result in a heat treated component with a skin that is much softer (or harder) than the rest of the component. To minimize these undesirable reactions, the source of the reactive elements, air, must be eliminated from the heat treating environment. Sometimes this is done by replacing the air in a heat treating chamber with a non-reactive atmosphere such as nitrogen, argon or other gas mixtures. This is often referred to as controlled atmosphere heat treating. Another alternative is to heat treat in a bath of non-reactive molten salt. However, these environments still contain some very low levels of residual impurities so metals heat treated in controlled atmosphere or salt usually exhibit a small amount of discoloration. In vacuum heat treating, most of the air and its reactive elements are removed from the heat treating environment. Most often, this is done by evacuating a tightly sealed heat treating chamber by means of a vacuum pumping system. While the pumping system cannot remove all of the air, it does remove so much that there is hardly anything left to react with the metal being heat treated. Some pumping systems are designed to remove more air than others. A furnace with a mechanical pumping system alone removes enough air that there will be no apparent surface reactions (discoloration) during heat treating of most common metals like stainless steels or tool steels. However, highly reactive materials like titanium and some superalloys (Inconel) will still discolor if heat treated in a furnace with mechanical pumping only. To process these types of materials without any discoloration, an oil diffusion pump is added to the pumping system. The diffusion pump will remove even more air from the heat treating chamber to provide a super-clean environment for processing. Diffusion pumped systems are also preferred for vacuum brazing furnaces. Vacuum furnaces must be properly maintained to avoid developing leaks. Even a small leak can be enough to cause contamination of the load during heat treating. It is also important that the baskets or fixtures used to contain the workload as well as the workload itself are properly cleaned before being placed in a vacuum furnace. Contaminants from dirty fixturing or workloads can burn off during heat treating causing discoloration of the workload and furnace hot zone. Even when proper cleaning precautions are employed, some amount of contamination will eventually build-up within the vacuum heating chamber. The recommended practice is to bake-out the empty furnace at regular intervals (perhaps once per week) to remove the contamination. The bake-out cycle involves heating the empty furnace to a temperature at least 100 F higher than its normal operating temperature and holding it at that temperature for a period up to 30 minutes. The heat causes the contaminants to burn off from the furnace internals and they are subsequently removed from the chamber by the vacuum pumping system.
Hardening of Steel by Heat Treatment -3- Steel alloys are among the most common materials processed by heat treating. Many general heat treating applications involve the hardening and tempering of a steel part to make it strong and tough in service. Hardening involves heating the steel to a pre-determined temperature, then cooling it rapidly. The first step in the hardening process is called austenitizing. In this step, the steel is heated to a temperature that changes the space lattice to a specific shape that creates a structure called austenite. The temperature to which the steel must be heated for austenitizing varies depending on the chemical composition of the steel. For example, landing gear steels like 4340 are generally heated to an austenitizing temperature of about 1600ºF. Tools steels such as D-2 and H-13 have to be heated to higher temperatures (1850ºF and higher) for proper austenitizing. The austenitizing temperature for any specific steel alloy is already well-known and can be found in any general heat treating guide. The transformation to austenite is not instantaneous so the steel must be held, or soaked, at the austenitizing temperature for a sufficient period of time. The length of time will depend on the type of steel and other physical factors like the size and section thickness of the part. The next step in the hardening process is called quenching. In quenching, the steel is cooled very quickly from the austenitizing temperature, which causes the space lattice to transform into a different shape that creates a structure known as martensite. Martensite is the structure that gives the steel its best hardness and strength properties. If the steel is not cooled quickly enough during quenching, the transformation to martensite will not take place. Each type of steel has a well defined critical cooling rate for complete martensite transformation. Some steels must be cooled far more quickly than others and have to be quenched in liquids (such as oil) to achieve the critical cooling rate. Others can be cooled more slowly by gas quenching or even air cooling and still achieve the critical cooling rate. The heat treater usually chooses the slowest method of cooling that will achieve the necessary critical cooling rate. This is because the faster the cooling method, the greater the risk that the steel will distort or crack during quenching. Martensite is a very hard structure but also very brittle. After quenching, the steel must be tempered to reduce the brittleness and make it tougher. Tempering involves re-heating the steel to a relatively low temperature. This causes a slight relaxation in the highly strained space lattice that makes up martensite. Besides hardening and tempering, there are other heat treatments that can be given to steels to produce different properties. For example, annealing involves heating the steel up to the austenitizing temperature then cooling it very slowly, often over many hours. This creates a structure that is very soft and pliable which is desirable if the steel has to be formed or machined. Normalizing is somewhat similar to annealing but involves faster cooling. Normalizing is used to refine the microstructure of the steel to improve machinability or the steel s response to further heat treatment. Stress relieving involves heating the steel to a relatively low temperature to remove internal stresses that may have been caused by forming or machining. Some types of steels undergo different changes during hardening that do not result in transformation to martensite. These are called precipitation hardening steels. Because of their chemical composition, some steels cannot be hardened at all. These include mild steels and austenitic stainless steels.
-4- Figure 2: Temperatures, times and cooling rates for heat treating metals have already been wellestablished by metallurgists and documented in records such as this transformation diagram. This information is readily available to the heat treater. What is Brazing? Brazing is another heat treating process used to join two or more base metal components by melting a thin layer of filler metal in the space between them. Bonding results from the intimate contact produced by a small amount of mixing of the filler metal and the base metal, without melting of the base metal. Brazing differs from welding because a welded joint is formed by melting of the base metal. Brazing is similar to soldering but, by definition, is performed at higher temperatures. In furnace brazing, the filler metal is pre-placed in the joint as a foil, or placed over the joint in the form of paste or wire. Joint clearances must be very carefully controlled and usually do not exceed.005. Capillary action draws the filler metal into the joint and holds it there. Vacuum furnaces are ideal for brazing applications. Braze alloy will not readily wet an oxidized or contaminated surface. This is why fluxes are usually needed for non-vacuum brazing processes such as soldering or torch brazing. The vacuum environment keeps all of the surfaces clean without the need for flux. What is a Partial Pressure? When certain chemical elements are heated to high temperatures in a vacuum, they can begin to vaporize. This means they will change from a solid directly into a gas, without first melting into a liquid. This change is not instantaneous but occurs gradually over a period of time. Elements more prone to vaporizing and the conditions of temperature and vacuum level under which vaporization will occur are well-defined by vapor pressure curves. The most common elements found in general heat treating or brazing applications where vaporization is a concern are chromium (present in high concentrations in many stainless steels and high temperature braze alloys) and copper, silver and gold (used in many brazing applications). When these metals vaporize from solid into gas, the gas will eventually condense onto the colder parts of the furnace such as the chamber walls or insulating shields in the hot zone. This may subsequently lead to contamination of other workloads and deteriorating vacuum levels.
-5- Figure 3: This chart shows the vapor pressure curves for some common elements found in metals and braze alloys. The area to the right of each curve indicates the conditions under which the element will vaporize. This information assists the heat treater in determining when a partial pressure should be used during the process. To prevent vaporization, the heat treater can introduce a partial pressure of a non-reactive gas such as nitrogen and argon. This is accomplished by first evacuating the furnace to its best, or ultimate vacuum level. Then a small amount of gas is introduced so that the heating chamber is partially pressurized from its initial ultimate vacuum level. The pressure is increased only to a level sufficient to prevent vaporization of any of the volatile chemical elements in the workload. Partial pressures are usually set in the range of 1000 microns, which is still very much less than atmospheric pressure. Hydrogen is another gas frequently used for partial pressure applications. Hydrogen differs from nitrogen and argon in that it is quite reactive with many elements, particularly oxygen. As a result, hydrogen creates a cleaning effect by reacting with and removing the oxides that can build up on the surface of metals. It is particularly useful as a partial pressure when brazing with copper or silver alloys. These elements develop natural protective oxides that need to be removed for good braze alloy flow. However, hydrogen is so reactive that it can become explosive under certain conditions. Vacuum furnaces designed for operating with hydrogen partial pressures include special safety devices to prevent any explosive reactions. Measuring Temperature During Heat Treating In any heat treating cycle, there are two important considerations concerning temperature: the temperature of the furnace hot zone which is generating the heat input, and the temperature of the actual workload. This is particularly important in vacuum furnaces where heating takes place primarily by direct radiation, which tends to be a slower process than other heating mechanisms such as convection or conduction. As a result, there are times in the heat treating cycle (particularly
-6- during heat up) when the load will be at a lower temperature than the furnace hot zone. This is known as temperature lag. Hot zone temperature is controlled and measured through sensing devices, known as thermocouples, located close to the heating elements. One thermocouple, the control thermocouple, is connected to a thermal process controller and transmits signals to control the amount of power directed to the furnace elements. The second is known as the overtemperature safety thermocouple and is connected to a separate controller that will shut off the power to the furnace if it unexpectedly overheats. Each of these can also be connected to a chart recorder to provide a permanent record of time and temperature data. These thermocouples are usually a platinum-platinum/rhodium combination known as Type S that retains accuracy under vacuum throughout a range between ambient temperature and 1480 C (2700 F). The location of these thermocouples within the hot zone is quite important to maintain good temperature uniformity. Workload thermocouples are usually attached directly to the load and monitored by the chart recorder. This makes it possible to record the exact time at temperature of the workload. Proper placement of these thermocouples will ensure the load receives the proper time at temperature by identifying the need for adjustments to correct temperature lag. Workload thermocouples are usually a chromel-alumel combination known as Type K. While this type is not recommended for repeated use at temperatures higher than 1204 C (2200 F), it is less delicate and much cheaper than Type S and can be supplied in a flexible construction ideally suited for attachment to a variety of load configurations. The locations where workload thermocouples are placed in the load can have a significant effect on temperature indications. The recommended practice is to use at least three workload thermocouples. One should be buried deep inside the load. The second should be placed at a midpoint between the center and outside of the load. The third can be placed on the surface or outermost limit of the load. Normally, the center of the load will require the longest amount of time to heat to the processing temperature. By monitoring all three workload thermocouples via the chart recorder, processing decisions can be made to ensure a sufficient soak of the entire load is achieved. Gas Quenching In a gas quench vacuum furnace, cooling of the workload is accomplished by backfilling the furnace chamber with a non-reactive gas, then circulating the gas throughout the workload. Nitrogen is an inexpensive quench gas that is very efficient in removing heat from the workload. However, there is an extremely small possibility that nitrogen will react with some metals during quenching so certain very critical heat treating applications require argon as a quench gas. Argon is truly inert, which means that it will not react at all with any other chemical elements. However, compared to nitrogen, argon is very expensive and not nearly as efficient at removing heat from the load. Another quench gas used occasionally is helium. Like argon, helium is completely non-reactive. It is also even more efficient than nitrogen at removing heat from the workload. Unfortunately, it is so expensive that it usually must be recovered for re-use after each quenching operation. The recovery systems are also quite expensive. The quench loop of a vacuum furnace consists of a blower, heat exchanger and manifold to distribute the quench gas evenly throughout the load. The quench cycle starts when the chamber is backfilled with the quench gas. The blower starts almost immediately and forces the gas through nozzles in the manifold and into the load. After passing through the load, the hot gas leaves the heating chamber through ports at the front and rear and flows up through the heat
-7- exchanger where it is cooled. It is then drawn back down through the blower and re-circulated back into the heating chamber. The vacuum furnace can be designed to backfill and quench at different gas pressures. By pressurizing, the quench gas is made more dense. A denser gas will remove more heat from the workload. The higher the pressure of the quench gas, the faster the cooling rate it can achieve. Standard furnaces are usually designed with quench pressures selectable at.85 and 2 bar. A bar is a unit of pressure approximately equivalent to atmospheric pressure or about 14.7 pounds per square inch. High pressure quench furnaces are designed for quenching at pressures up to 6 or 10 bar, or even higher. However, these furnaces are also capable of quenching at lower pressures as well. The fast cooling rates that can be achieved by high pressure gas quenching are most important in furnaces used for hardening steels. The development of high pressure gas quenching has allowed the heat treater to use vacuum furnaces to harden many steels that could be previously hardened only by quenching in liquids such as oil or molten salt. Figure 4: As quench pressure increases, cooling times decrease. This chart shows the relative improvements in cooling time when increasing quench pressure from 1 bar through 5 bar. Typical Vacuum Processing Cycle Whether heat treating or brazing, certain common steps make up all vacuum processing cycles. Usually, these steps are programmed into the furnace controller so they can be performed automatically. The typical vacuum heat treat cycle consists of the following steps: (i) (ii) Place the workload in the furnace. Attach workload thermocouples, if necessary. Close and secure the door to seal the chamber. Evacuate the furnace to a preset level. The vacuum level at the beginning of the cycle does not need to be the lowest level that the pumping system is capable of reaching. Evacuation continues throughout the cycle and better or higher vacuum levels will be achieved as the cycle continues. The vacuum instrument in the furnace control system can be programmed to ensure proper vacuum levels are achieved before the workload reaches higher temperatures.
-8- (iii) (iv) (v) (vi) (vii) Begin heating. Standard heating rates are usually between 20 F/minute and 35 F/minute. Slower rates are used for heavier loads, faster rates for lighter loads and for many brazing cycles. Heat to a stand-off temperature 100 F to 200 F below the final processing temperature and soak for up to 60 minutes. This allows the temperature within the entire workload to stabilize. Sometimes, more than one stand-off soaks are used. Heat to the final soak temperature and hold for the prescribed period of time. Backfill the furnace with a quench gas (such as nitrogen). Activate the quench blower and cool the load down to room temperature. Equalize the chamber pressure to atmospheric pressure, open the door and remove the workload. Heating in vacuum is not very efficient at temperatures less than 1000ºF. In lower temperature ranges, the temperature of the workload will lag behind the temperature of the heating elements. Furthermore, the workload will tend to heat from the outside toward its centre so the centre of the workload can be significantly colder during the early stages of the heating cycle. The stand-off soak allows the temperature of the workload to catch up to the temperature being produced by the heating elements. Proper positioning of workload thermocouples lets the furnace operator know when temperature variations exist within the load. Workload thermocouples should be placed in the centre and outer areas of the workload or near the thickest and thinnest crosssections of pieces within the workload. Because heat transfer within a vacuum occurs mostly by radiation of energy from the heating elements to the workload, it is very important that individual pieces in the workload are correctly positioned. The individual pieces should be properly spaced to maximize direct heat radiation from the elements. If placed too closely together, one piece may shield another, which will slow down the heating rate and increase the likelihood of larger temperature variations within the load. Proper spacing is also important for fast cooling. The quench gas must be free to circulate over the entire surface of each piece within the workload. Heat Treating Quality Control After a component is heat treated, there are a number of quality control tools available to check that the heat treating was performed properly. The simplest and most common is a hardness test. The hardness of a metal provides a very good indication of its internal microstructure and related mechanical properties. In most cases, a Rockwell hardness tester is used to measure hardness by making a small, almost unnoticeable, indentation in the part. As a result, this test can be used to check finished parts. Other quality control tests involve sectioning a heat treated part or sample to inspect its internal structure or breaking of specially designed test specimens that are heat treated with the parts. Some examples of the latter test include tensile testing and impact testing.