Posted on November 2, 2020
What is Heat Treating?
Heat treating refers to thermal and metalworking processes that alter the chemical and physical properties of a material. Generally, metal is heated and cooled under tight controls.
The heating is done at extreme temperatures to achieve the desired results. Some of the common heat treatment techniques include case hardening, annealing, tempering, strengthening, quenching, and normalizing. This process is used in industries like automotive, construction, aerospace, military, and oil & gas. Also, cutting tools, camshafts, fasteners, all depend on heat treating.
The basics of heat treating
While heat treating is commonly used where cooling and heating is done, it can also be used in manufacturing processes like welding or hot foaming. And depending on your needs, the process can make the parts harder or softer to improve permeability. Still, it can protect the parts that would otherwise corrode.
Heat treating involves the use of heat at different levels. Typically, the process is as follows:
- Heating an item at a predefined temperature
- Holding that temperature for a specific amount of time
- Cooling the item
Depending on the metal you’re working on, the temperature can be set as high as 2400 degrees F. The time may vary from a few seconds to a couple of hours. Of course, the materials can undergo the cooling process differently. Some are cooled fast, and others slowly. Keep in mind that 90% of the parts are quenched in polymers, gases, oil, and water.
What is the value of heat treating?
This method has impacted different industries. It’s closely linked to the manufacturing of steel products – forged, cast, welded, stamped, extruded, drawn, or machined. It’s also a vital step in manufacturing non-ferrous products. For example, brass is treated to increase strength and to prevent cracking. Similarly, titanium and aluminum products are heated to improve strength.
Preheat and Post Weld Heat Treat
Using extreme heat, thermoplastics and durable metals can be joined together to repair machinery. While welding is seen as a solution to many complex problems, you may not get the best results once the heat subsides. If you’re not careful, you could cause undue stress to the finished product. This is where pre-heat and post-weld heat come in.
The preheating process helps to minimize the temperature between the base material and welding arc. Since hot materials expand before cooling, there’s a variation of temperature between the cool base material and the molten weld pool. These internal stresses help to minimize the temperature differences.
In any heat treating process, the second step is cooling. It helps to reduce the hardness in the affected area. These characteristics are important for materials that are susceptible to hardness when exposed to high temperatures. The pre-heat process is applied to materials such as high-carbon steel, alloy steel, and cast iron.
As the weld hardens, hydrogen is removed to prevent cracking that may occur. The next step is to introduce heat in the weld area. Through pre-heating, you can be sure you’ll use less welding in the welding arc.
Preheating is beneficial for alloys with highly restrained joints and materials that tend to be a little bit brittle. It also works great for materials with high carbon levels, which are prone to cracking.
Post weld heat treatment
Post-weld heat treatment (PWHT) involves altering the physical and chemical properties of a metal. It’s also referred to as controlled heating and cooling of metal. Unlike other heat treating methods, PWHT introduces certain changes in properties. Also, it leaves the steel free from harmful internal stresses.
Another reason why this process is unique in the fact that it removes the external brittle structures that follow after cooling. To some engineers, this process has a very precise meaning. That being said, PWHT is carried out for three fundamental reasons:
- To achieve the required chemical properties
- To maintain tolerances of metal and achieve the right dimensional properties
- To reduce the risk of service problems
- To minimize residual stress in the welded component
It’s worth mentioning that this process can also bring changes to the microstructure of the metal and cause the welded material to suffer residual stress. During the heat treating process, PWHT may cause irreparable damage when used incorrectly.
How to perform a post-weld treatment
If materials are welded and cooled too quickly, a fracture may occur. Besides that, corrosion may occur due to extreme heat. With this in mind, welding should be done by following the code of the material at hand (EN 13445 and BS PD 5500). The steel should then be cooled in a way that no harmful gradients can occur.
The theory behind this process is straightforward. As the temperature of the metal increases, the yield strength increases. This, in turn, allows the residuals stresses to be redistributed to the parent metal. Finally, the temperature is cooled to ensure there are no harmful thermal ingredients.
If you don’t know the purpose of the welding job, it can be tough to decide whether you should carry out preheating or post welding. So before you do your welding job, you should consult a professional in heat treating to evaluate the method that suits you best.
Since PWHT helps to avoid changes after cooling too fast, there a few things to keep in mind.
- The thicker the weld, the higher the residual stress
- When you control the cooling temperature of materials, you could reduce the strength rather than increase it.
- Welding two pieces together can change the internal structure
Resistance heating is a form of heat treatment that has a wide range of applications like annealing and hardening. It’s also common in domestic and commercial cooking, stoving of enamelware, and drying & baking of potteries. The temperature can go up to 1000 degrees F in ovens that use wire resistances for heating elements. There are two main methods of resistance heating:
- Direct resistance heating
- Indirect resistance heating
Direct resistance heating
In this heat treating method, the material to be heated is taken as resistance as the current passes through it. The charge can be in the form of liquid or powder. Then, two electrodes are immersed in the charge (single-phase AC or direct current). If you want to heat high pieces of metal, a resistance powder is sprinkled over the surface to avoid a short circuit.
As the current flows through the charge, heat must be produced. However, there’s a drawback when using this method of heat treating. It’s common in electrode boilers and salt bath furnaces.
Indirect resistance heating
This is another common heat treating method where the current passes through the wire of a high-resistance material. The heating element is then produced to the charge through conduction, radiation, and convection. If the heat transfer is done through conduction, then the resister should come into contact with the charge.
The enclosure (heating chamber) is required to transfer the heat through convection or radiation. Since this method is best suited for industrial purposes, the heating element should be kept in a cylinder. With this kind of arrangement, you can get automatic temperature control.
Indirect resistance heating can be employed in both domestic and commercial cooking. More specifically, it can be used in immersion water heaters, room heaters, and room heaters.
How to use resistance heating ovens and furnaces
Since resistance heating is a common type of heat treatment, it can be used to dry varnish coatings, hardening synthetic materials, baking potteries, vulcanizing, and domestic heating.
In most cases, medium temperature furnaces operate at 300-1500 degrees F. This is what you need to for normalizing and annealing non-ferrous metals.
Resistance heating is also used in firebricks since the insulating material is supported on the metal framework. And based on individual circumstances, the heating elements are mounted to the top, bottom, or sides.
Since the insulating furnace consists of a layer that surrounds the casing of a steel plate, the proportions of the heating chamber must suit the character of the charge. Still, the nature of the material required for insulation is determined by the inner temperature of the inner surface.
Apart from the mechanical considerations of the design, heat transfer is the major mode of heat to be used. The function of the chamber is:
- To control the cooling rate of charge
- To control the distribution of heat
- To confine the atmosphere that surrounds your charge
Just like other types of heat treating, the materials used in resistance heating must possess specific characteristics. This includes:
High melting point
When the melting point of the material in question is high, the charge can be treated to a high temperature.
Free from oxidation
The material for a heating element should withstand the set temperature without being oxidized. If you don’t follow this simple heat treating tip, you should be prepared to replace the material more often. The common materials used for heating are:
Induction heating is a type of heating used to harden or soften metals. In modern manufacturing processes, it offers an attractive combination of consistency, speed, and control.
While induction heating dates back to the 1920s, it developed rapidly in the hardening of metal engine parts. Today, this type of heat treating is commonly used in manufacturing techniques that emphasize quality control.
So, what makes induction treating unique? An open flame is directly applied to the metal part to circulate electrical currents. This type of heat treating can be likened to the unique characteristics of radiofrequency energy. Since the energy is transferred through electromagnetic waves, some parts don’t come into contact with the flame.
How induction heating works
Induction heating uses the basic principles of electricity. When the primary current of a transformer is applied to the alternating current, a magnetic field is created. In an induction heating setup, the power supply sends AC, and the part to be heated is placed inside an inductor. Then, the inductor works as the primary transformer and the part to be heated becomes a secondary circuit.
The electric current generates localized heat without any contact with the part of the inductor. Since the heating occurs with both magnetic and non-magnetic parts, there’s a scientific formula that can help to calculate the heat produced by the electrical current.
What makes this type of heat treating unique is that electrical heat is produced within the magnetic parts. Then, the electric materials offer resistance to the rapidly changing resistance fields. This kind of resistance produces friction that turns into heat.
In the process of heating the material, there should be contact between the metal part and the inductor. Just like any other method of heat treating, the efficiency of induction heating will depend on several factors like:
- The design of the inductor
- Characteristics of the part
- The capacity of the power supply
- Amount of temperature change
The magnetic field required to heat a particular part can easily be calculated. Since the design is one of the important parts of the overall system, a well-designed inductor will give a proper heating pattern. This maximizes the induction heating power and allows easy insertion of the part. A well-designed inductor provides a proper heating pattern and maximizes efficiency.
Degree of temperature change required
The efficiency of any induction process will depend on the temperature change you need. A general rule of thumb is to do a wide range of temperature changes to meet the desired degree of temperature change.
The capacity of power supply
First, you must determine the amount of power required to heat a specific part. That way, you can know how much power should be transferred to the workpiece. A heat loss through convection, radiation, and conduction should also be considered.
Characteristic of the part
The induction heating works with conductive metals as well as plastics. Some metals can be heated indirectly by first heating the metal and transferring the heat to the non-conductive material. The size of the induction material also matters. Therefore, small and thin parts should be heated all the way through.
Wrapping it up
If you want to bring the desired change in the physical properties of metals, heat treating can be a sure bet. This is what metalworkers rely on to ensure they meet the needs of their customers. Keep in mind that heat treating affects different aspects of metal like toughness, hardness, formability, and strength.