Most steels need to be tempered at about 450°F for maximum usable hardness but every steel is slightly different. To soften steel so that it can be cold worked and machined is called annealing. To anneal steel is is heated to slightly above the hardening temperature and then cooled as slow as possible.
It starts with heated metal that is air cooled. This simple act, if heated to an exact temperature range, can create a more pure, hard metal. It's often used to create steel that is stronger than annealing the metal, but also creates a less ductile product. So, heat can indeed make metal weaker.
Set your steel directly on the oven rack or on a baking sheet. Let your oven heat the piece of steel. During the tempering process, the steel heats up enough to soften the alloys inside to make it less brittle. If you need to use a blow torch, focus the flame tip on the area you want hardened.
To make steel harder, it must be heated to very high temperatures. The final result of exactly how hard the steel becomes depends on the amount of carbon present in the metal. Only steel that is high in carbon can be hardened and tempered.
Quenching improves a metal's performance by rapidly cooling the heated metal, thereby altering its molecular structure and increasing its hardness.
Nitriding is an excellent method of controlling corrosion, as well as wear and fatigue, in metals.
Carburizing and Nitriding are the two techniques used in differential metal structure hardening process. The main difference between carburizing and Nitriding is that in carburizing, carbon is diffused to the steel surface whereas, in Nitriding process, nitrogen is diffused to the steel surface.
Flame hardening is a surface-hardening method that involves heating a metal with a high-temperature flame, followed by quenching. It is used on medium carbon, mild or alloy steels or cast iron to produce a hard, wear-resistant surface.
The steel is heated to a critical temperature above 30-50℃. After a while, the heat treatment process cooled in the air is called normalizing. Compare quenching with annealing and normalizing, the main difference is the quick cooling, the purpose is to obtain martensite.
Explanation: Nitriding is generally not preferred for plain carbon steels. This is due to the formation of iron nitrides formed to a considerable depth below the surface of the steel. This causes embrittlement of the material.
The temperature of the process is in the range of 925°F – 975°F which is below the tempering temperature of the steels being processed. Hence there is little or no distortion, only about 0.0005″ growth due to the nitrogen diffusion into the part. So no post heat treatment machining is needed.
During nitriding the volume of the component increases by 3% of the layer thickness. Adhesion is very good.
Advantages and Disadvantages of Nitriding
- Better retention of hardness at elevated temperatures.
- Greater fatigue strength under corrosive conditions.
- Less warping or distortion of pans treated.
- Higher endurance limit under bending stresses.
- Greater resistance to wear and corrosion.
- Greater surface hardness.
Drawbacks of induction hardeningDistortion: Distortion levels do tend to be larger in induction hardening than they are in processes such as ion or gas nitriding as a result of the rapid heating and quenching process and the martensitic transformation that occurs as a result.
All steel has to be treated in order to be used in commercial products. The heat treatment of steel generally always involves annealing, quenching, and tempering.
Carbon strengthens steel and gives it the ability to be hardened by heat treatment. As carbon-rich austenite cools, the mixture attempts to revert back to the ferrite phase, resulting in some areas having an excess of carbon.
When carbon steel is hardened, the steel must be cooled to under 1000°F in less than one second. But, once you add alloys to the steel and increase the effectiveness of the carbon, you increase that time limit beyond one second.
Metal BendingYou can make simple bends in iron and steel using nothing more than a torch to heat and soften the metal. With a soapstone pencil, mark a line across the stock to indicate the center of the bend. Clamp the stock in a vise. Light the torch and adjust it to a neutral flame.
Metal fabricators use annealing to help create complex parts, keeping the material workable by returning them close to their pre-worked state. The process is important in maintaining ductility and reducing hardness after cold working. In addition, some metals are annealed to increase their electrical conductivity.
Stages of Heat Treatment
- The Heating Stage.
- The Soaking Stage.
- The Cooling Stage.
The heat treatable steels are classified under 4 main groups based on their chemical compositions:
- Unalloyed heat treatable steels.
- Manganese alloy hot treatable steels.
- Chrome alloy hot treatable steels.
- Chrome-molybdenum alloy hot treatable steels.
There are five basic heat-treating processes: hardening, tempering, annealing, normalizing, and case hardening. Although each of these processes brings about different results in metal, all of them involve three basic steps: heating, soaking, and cooling (Fig. 1.45).
Austenitic stainless steels cannot harden via heat treatment. Instead, these steels work harden (they attain hardness during their manufacture and formation). Annealing these stainless steels softens them, adds ductility and imparts improved corrosion resistance.
The hardness of the nitrided layer can be higher than that achieved by carburising and is in the range of 800–1200 HV.
The maximum hardness of a steel grade, which is obtained by hardening, gives the material a low toughness. Tempering reduces the hardness in the material and increases the toughness. Through tempering you can adapt materials properties (hardness/toughness ratio) to a specified application.
For instance, mechanical properties of austenitic stainless steel like resistance to wear can be significantly augmented and the surface hardness of tool steels can be doubled. A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations.
Nitrided parts will typically be matte gray in color. AHT's trademarked process for added corrosion resistance, UltraOx®, will result in a sleek black finish as shown to the right.
4140 hardened and tempered bar can also be successfully nitrided, giving a surface hardness of up to Rc 60. Nitriding is carried out at 490 oC - 530 oC, followed by slow cooling (no quench) reducing the problem of distortion. Parts can therefore be machined to near final size, leaving a grinding allowance only.
In carbon steel and low-alloy steel, the maximum carbon is about 2.0%; in high-alloy steel, about 2.5%.
Which of the following processes will one use on hardened steel to reduce brittleness? Explanation: Tempering is a process of heating to lower critical temperature, then maintaining it at temperature followed by slow cooling to relieve stress and reduce brittleness, increase ductility.
Untempered martensite is a strong, hard, brittle material. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite.
Age hardening, also known as precipitation hardening, is a type of heat treatment that is used to impart strength to metals and their alloys. The metal is aged by either heating it or keeping it stored at lower temperatures so that precipitates are formed. The process of age hardening was discovered by Alfred Wilm.
