Induction Heating: Everything You Wanted to Know, But ...

Question of the Month: What is the difference between auto-tempering and self-tempering?

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Answer: Sometimes, the terms auto-tempering and self-tempering are incorrectly used interchangeably. Here, we will explore the differences between these two terms (Materials in this article have been adapted from the 2nd Edition of the Handbook of Induction Heating by V. Rudnev, D. Loveless, and R. Cook, CRC Press. CRC Press has granted permission to publish these materials).

Auto-tempering

Martensitic transformation occurs over a temperature range between the Ms (martensite start) and Mf (martensite finish) temperatures. This range depends on the steel's chemical composition and, from a practical standpoint, cannot be altered by varying the quench severity. In plain carbon steels, the Ms and Mf temperature range is directly related to carbon content. For plain carbon steels with carbon content between 0.2% and 0.5% C, Ms temperatures typically range from about 300°C (572°F) to 450°C (842°F). Consequently, freshly formed martensite will be promptly exposed to tempering temperatures, allowing for potential softening. This phenomenon is known as auto-tempering. The degree of auto-tempering is more apparent with a reduction in quench severity, an increase in Ms temperatures, and the mass of the heated material (for example, through heating versus surface heating), as well as whether interrupted quenching is applied [1]. Alloy steels exhibit auto-tempering to a lesser extent compared to plain carbon steels.

Self-tempering

The principle of self-tempering (also referred to as slack-quenching) can be illustrated through the example shown in Fig. 1 [1], which displays the results of numerical computer modeling of induction surface hardening of a medium carbon steel solid shaft (50 mm/2-in. diameter) in a normalized condition using a frequency of 16 kHz. The required nominal case depth is 2.5 mm.

During the initial stage of induction heating, intensive heating of the surface and near-surface occurs. After 3.5 seconds of heating, the surface and 2.5-mm-thick subsurface layer required for hardening reach suitable temperatures for austenitization, considering the non-equilibrium phase transformation brought about by rapid heating. A short dwell (0.5 seconds) is applied to mitigate thermal shock during the initial stage of quenching.

The temperature at the center (core) of the shaft does not increase significantly (less than 100°C) during the heating and dwell cycles. Several factors contribute to this, including a pronounced skin effect, high power density, and a short heating time that prevent a larger amount of heat from being conducted from the surface to the core.

During the initial quenching stage, the high temperature of the surface layer begins to decrease rapidly. After 2 seconds of spray quenching, the surface temperature is sharply reduced to around 210°C (410°F). The maximum temperature reaches about 400°C (752°F) approximately 10-12 mm beneath the surface. Note: The temperature at the core of the shaft continues to rise during the first 6 seconds of quenching.

After 6 seconds of quenching, the surface temperature drops below 100°C (212°F); however, a significant amount of heat remains within the shaft (the temperature at the core exceeds 300°C (572°F) with an average of about 225°C (437°F)). If, at this point, the supply of the quenchant is cut off, the part's surface will begin to heat up again due to the heat accumulated inside the workpiece.

After 5 seconds of soaking (heating power and quench are not applied), the surface temperature rises to about 215°C (419°F) and the core temperature is around 260°C (500°F). Thus, with proper processing conditions, this retained heat can be utilized to temper the workpiece.

In many instances involving plain carbon and low alloy steels for automotive applications, self-tempering temperatures (if applied) generally do not exceed 250°C (480°F) and typically range between 180°C (360°F) to 220°C (430°F).

Benefits of Self-tempering

Self-tempering provides several notable benefits [1]:

• It eliminates the need for an additional operation, incorporating self-tempering into the hardening process. As a result, both capital equipment costs and total cycle times are reduced, making it financially attractive.
• The time gap between hardening and tempering stages is virtually eliminated, which is significant since a lengthy delay may lead to delayed cracking.
• Since self-tempering utilizes residual heat retained after hardening, there is no requirement for additional energy input during tempering, making it highly energy-efficient. This reduction in energy consumption applies not only to the reheating stage but also to cooling.
• There are tangible savings in shop floor space, as there is no requirement for extra room for tempering equipment.

Although these factors make self-tempering appealing, several measures must be taken to ensure the self-tempering process is executed correctly. Despite its advantages, self-tempering has limitations that hinder its broader application in the industry, making furnace tempering and induction tempering more popular. Some of these limitations include:

• Residual heat must be closely controlled, and the energy generated within the part must be monitored to ensure a consistent heat production. Modern induction technology allows precise energy level monitoring. It is, however, more challenging to control quench severity with the same precision, especially with complex geometries. Factors such as quench time, flow rate, temperature, concentration, and cleanliness of the quenchant also need to be monitored for stability in thermal conditions post-quenching. Variations in quench severity are influenced by both the quenchant's actual condition and the surface condition of the workpiece, including surface roughness and residues from prior operations. The impacts of these variations may not present dramatic repercussions during the first two quenching stages but can significantly affect self-tempering repeatability during the third stage.
• Self-tempering may be more suitable for static heating, single-shot heating, and to a lesser extent, horizontal scan hardening or continuous/progressive hardening applications. It is not advisable to use it in vertical scan hardening due to uneven cooling conditions resulting in variations of residual heat in the upper and lower regions of the workpiece.
• Self-tempering is easier to implement with simple geometries (e.g., straight shafts). Localized variations in heating and quenching may occur in complex components, potentially leading to significant deviations in residual heat.
• Some steels and cast irons exhibit relatively low Ms temperatures, meaning that upon completion of martensite formation, there may not be enough retained heat for sufficient self-tempering.
• Controlling residual heat is more difficult with small components (e.g., wires, thin-walled tubes, small rods). As such, self-tempering is easier with parts possessing sufficient mass. Conversely, larger workpieces with high diamater-to-thickness ratios might not suit self-tempering due to the cold core's overpowering cold-sink effect.
• Self-tempering should be avoided during profiled hardening (such as contour hardening for gears), as neighboring mass variations can lead to nonuniform tempering effects. Uniform heat storage and heat sinks in adjacent areas are essential for consistency during self-tempering.

The challenges mentioned above restrict self-tempering's widespread use in the industry, thus making furnace/oven and induction tempering favored alternatives. However, there are specific applications where self-tempering has been successfully combined with induction tempering, leveraging the advantages of both processes. For instance, combining self-tempering with multi-pulse induction tempering is effectively used in non-rotational crankshaft hardening (SHarP-C technology). In this method, crankshaft journals undergo stationary heat treatment. For most automotive crankshafts, it typically takes about 3 to 4 seconds to austenitize a journal surface layer for hardening using frequencies ranging from 10 to 30 kHz, depending on the crankshaft's specifications and the required case depth. Following this austenitization, quenching occurs for only 4 to 5 seconds, followed by 3 to 5 seconds of the first soaking that accomplishes the initial stage of self-tempering. This is succeeded by low-power induction tempering for approximately 3 to 5 seconds, leading to a second soaking and induction tempering cycle. The process can be repeated to achieve the desired tempering condition, thus optimizing the tempered structure. SHarP-C technology, along with the intricacies of induction tempering, are thoroughly discussed in reference 1.

Dr. Valery Rudnev, FASM
Director, Science & Technology
Inductoheat Inc
www.inductoheat.com

Reference

1. V. Rudnev, D. Loveless, and R. Cook, Handbook of Induction Heating, 2nd Edition, CRC Press.

Beginning in July, the Professor Induction column initiated a new article series titled Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask. The most frequently asked questions regarding various aspects of induction heating and heat treatment will be reviewed and explained. All are welcome to send inquiries to Dr. Rudnev at . Selected questions will be answered in this column without revealing the author's identity unless specific permission is granted.

Induction forging is a method for heating metal to the forging temperature using induction heating, followed by forming the metal with a die. Induction forging has numerous advantages over traditional forging technologies, including:

Faster heating cycles: Induction heating can achieve the forging temperature much quicker than conventional methods like gas or electric furnaces, leading to significant productivity improvements.

Precise temperature control: Induction heating enables very precise temperature regulation. This precision is crucial for ensuring that the metal is heated to the correct temperature for forging. Overheating can weaken the metal, while underheating can complicate shaping.

Uniform heating: Induction heating ensures even metal heating, resulting in more consistent forgings, which is critical for applications requiring strength and precision.

Reduced defect scale: Induction heating generates less scale than traditional methods. Scale forms a hard, brittle layer on the metal's surface during heating, weakening it and complicating machining.

Improved surface finish: Induction heating enhances the surface finish of forged parts because the even heating reduces the formation of scale and other defects.

Reduced energy consumption: Induction heating is more energy-efficient than traditional methods since it generates heat directly within the metal rather than transferring it from surrounding air.

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Overall, induction forging is a highly adaptable and efficient technique with several advantages over earlier methods. It serves as an extremely beneficial tool for manufacturers across various industries, including aerospace, automotive, and medical components.

Here is a step-by-step overview of how induction forging works:

* The metal to be forged is placed within a coil connected to an induction heating machine.

* An alternating current flows through the coil, creating a magnetic field.

* The magnetic field induces an electric current in the metal, causing it to heat up.

* The metal reaches its forging temperature, typically between 1,100 and 1,200°C (2,010 and 2,190°F).

* The metal is then shaped using a die.

* The forged part is subsequently cooled.

Here are additional advantages of induction forging:

Reduced environmental impact: Induction heating produces less pollution compared to standard heating methods such as gas or electric furnaces since no fuel is needed to heat the metal.

Enhanced safety: Induction heating is a safer method than older techniques, as it poses no fire or explosion risk and operators are not exposed to hazardous gases.

Increased flexibility: Induction heating can accommodate various materials, including steel, aluminum, and titanium, and can be used for metals of diverse shapes and sizes.

These advantages make induction forging an appealing option for manufacturers seeking more efficient, environmentally friendly, and safe methods of heating metal.

Induction forging is a versatile process capable of forging a broad range of metals, such as steel, aluminum, titanium, and copper. The process is also utilized for forging assorted parts, including gears, shafts, and stampings.

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