What is induction hardening? Drawing instructions and quality control basics that go directly to practice

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In designing linear shafts, many designers wish to harden the surface to increase wear resistance.　　However, problems frequently occur in the field, such as seizure cracking due to inadequate drawing instructions or assembly difficulties due to unexpected distortion.High-frequency quenching is a heat treatment to solve these design issues.　However, there are only a limited number of places where one can systematically learn how to give such instructions.

While general websites often provide only a superficial glossary of terms, this article combines the technical basis of Japanese Industrial Standards (JIS) with practical data from manufacturers.　　In addition to my design experience, I have thoroughly researched and compiled the latest technical data, so I have covered specific calculation bases and quality control intuitions not found on other websites.

We will organize the basics of the hardening process by induction heating based on the principles of electromagnetic induction, and finally, we will explain the process from consideration of post-processing and strain correction, taking into account bending and residual stresses.　We hope that by reading this article, you will be able to give optimal heat treatment instructions with confidence.

What is high-frequency induction hardening?

Physical principle of rapid heating by induction heating

When only the "surface" of a machine part needs to be hardened, induction hardening is the most efficient option.　　The core of this technology lies in induction heating using the electromagnetic induction phenomenon. When a high-frequency current is passed through a coil, a magnetic field is generated in the surrounding area, inducing eddy currents on the surface of the steel material placed in the coil.　　These eddy currents are converted into heat by the material's own electrical resistance,Instantly raises surface to austenitizing temperatureThe first is to make them do it.

https://www.youtube.com/@earlybirdjun　The information is borrowed from the following website.

Unlike the method of heating the entire component in a furnace, heating is completed in a very short period of time, just a few seconds.　　Therefore, it is possible to harden only the necessary areas with pinpoint accuracy while minimizing thermal effects.　　After heating, the steel is quickly quenched with water or an aqueous solution to change its structure to hard martensite.The inside of the material is not fully heated, so the suppleness of the original material can be retained, which is a major feature.It is.

Influence of surface skin effect on hardened layer depth and frequency control

In order to burn to the desired depth, one must understand a physical phenomenon called the epidermal effect.　　Alternating current has the property of flowing over the surface of a conductor, and the degree of its concentration depends on its frequency.The higher the frequency used, the more the current is concentrated in a very shallow layer on the surface; conversely, the lower the frequency, the deeper the heat can penetrate.

For example, depending on the shaft diameter of the linear shaft and the required loading conditions, the equipment will select this frequency appropriately and adjust the depth.　　Smaller parts are generally burned shallowly at a higher frequency, while large shafts with large loads are heated to a deeper depth at a lower frequency.

Comparison of effective hardening layer depths in JIS standard metal material (round bar) (standard)

*The above is a rough guide, so if you need to decide on details, please consult with a heat treatment shop.

Knowing this principle, designers can predict the impact of unreasonable depth specifications on manufacturing costs and quality.

Reference: Characteristics of S45C and SUJ2 and criteria for their use

The quality of the finish of induction hardening depends on the amount of carbon contained in the steel.In Japanese design practice, the most standard selection is S45C, a medium carbon steel.　　This material has a very good balance between workability and cost, and is widely distributed as linear shafts for general conveying equipment and automatic machines.

On the other hand, SUJ2, a high-carbon chromium bearing steel, is selected for parts requiring higher wear resistance, such as machine tools and high-precision slides.　　While SUJ2 can achieve very high hardness by quenching, it should be noted that its high carbon content increases the risk of quench cracking.　　The attainable hardness and applications of each material are summarized in the table below.

Table 1: Comparison of properties of typical steels for linear shafts

Reference source: Sanyo Electronics Co.https://www.yakiire-netsusyori.com/kakou/koshuha.html）

Advantages of applying tempering materials that contribute to quality stabilization

Consider specifying tempering materials as a method to ensure stable quenching quality.Tempering is a process in which the steel is quenched and tempered in advance to prepare a uniform and fine solvite structure.　　Since there are variations in the structure of raw material, induction hardening with short heating time tends to cause insufficient hardness and uneven hardening.

Using the temper as the base material stabilizes the reaction during induction heating, making it easier to ensure that the effective hardening layer depth is obtained as designed.　　The core is also stronger, which has the advantage that the surface hardened layer is less likely to delaminate even when subjected to strong loads.　　In machine design where long-term accuracy and durability are important, the presence or absence of this pretreatment is an important point that differentiates reliability.

What is high-frequency quenching for proper design?

Index for determining effective hardened layer depth and critical hardness

When the hardening depth is indicated on the drawing, the reference is the effective hardening layer depth.　　I have summarized the effective hardening layer depths as a reference for each material in the previous table,This depth is defined in JIS G 0559 as the distance from the surface to the "limit hardness".　　The limit hardness is defined by the carbon content of the steel, and disregarding this will cause confusion in the inspection process if the instructions are ignored.

For example, for steels with a carbon content of 0.43% to 0.53%, such as S45C, 450HV (45HRC)Ultimate hardness　It is common practice to measure the value as　　In addition, by agreement between the delivery parties, the following formula is used to calculateIn some cases, the limiting hardness is calculated.

Hlimit = 0.80 × Hmin

WHEREAS.Hmin is the minimum surface hardness required　refers to a "numerical value" that is used to indicate the design intent.　　This numerical-based instruction makes it possible to accurately convey the design intent to the machining shop.

Table 2: Carbon content and limit hardness specified by JIS G 0559

Design for core toughness and difference from carburizing and quenching

When considering high-frequency quenching, carburizing and quenching are often compared.　　Carburizing and quenching is a method of diffusing carbon on the surface of low-carbon steel, but since the entire piece is heated for a long time, it tends to cause large deformation.　　In contrast,High-frequency quenching uses self carbon to locally heatThis makes it very good at maintaining the suppleness (core toughness) of the core while hardening only the surface where necessary.

For this reason, induction hardening is overwhelmingly advantageous for long linear shafts and crankshafts that require hardness only in certain areas.　　On the other hand,Carburizing and quenching has the advantage when uniform hardening of details of complex shapes is required.　　After understanding the characteristics of each, a choice must be made based on a balance of cost and delivery time.

HQI-HT drawing instructions based on JIS standards

When giving heat treatment instructions on drawings, it is advisable to use the processing symbols specified in JIS B 6912.　　The process of tempering in a furnace after induction hardening is described as HQI-HT.　　The tempering process is essential to relieve internal stresses caused by quenching, remove brittleness, and impart toughness.

Instead of simply stating "induction hardening," please include this symbol along with the hardness range and effective hardening layer depth.　　Also,End threads and other areas where hardening should be avoidedTherefore, clearly indicating the scope with dimensional lines will prevent problems in practice.

The use of appropriate symbols is the first step toward quality reproducibility.

Table 3: Symbols of high-frequency heat treatment according to JIS B 6912

Mechanism of Bending and Residual Stress

Distortion associated with heat treatment may be one of the most annoying problems for designers.　　In high-frequency quenching, in addition to thermal stress caused by rapid heating and cooling, transformation stress occurs due to volume expansion as the microstructure changes to martensite.The non-uniform action of these forces results in significant bending, especially in long parts.

The bend is a visible change, though,Inside them, mighty residual stresses lurk.　　While remaining stress in the compressive direction on the surface helps improve fatigue strength, it can also cause the part to warp again if the balance is lost during subsequent polishing or other processes.　　It is necessary to anticipate these shape changes from the design stage and provide an appropriate margin that leads to the subsequent finishing process.

What is high-frequency quenching to prevent problems and make the most of it?

Ensure shaft straightness by correcting distortion

After quenching, parts are often not usable as they are due to lack of precision, so distortion correction is performed.　　This is a process in which pressure is applied in the opposite direction of the bend using a press, etc., to bring the runout of the shaft within tolerances.　　Skilled workers repeatedly make fine adjustments while checking straightness with a dial gauge.

However, since unreasonable straightening can cause microcracks, care must be taken to avoid extreme differences in wall thickness at the shape design stage.　　It is also known that even if straightened temporarily, distortion returns after a lapse of time or processing.　　Therefore, low-temperature tempering for stabilization after straightening is an effective measure to maintain quality.

Appropriate setting of the polishing allowance essential for the finishing process

After induction hardening, linear shafts are ground to finish the outside diameter to a specified accuracy.　　Here, it is important to set the grinding allowance.　　To account for expansion and bending due to quenching and runout due to straightening, it is necessary to machine the part thicker than the finished size in advance.

In general, one guideline is to allow a margin of 0.3 mm to 0.5 mm in diameterIf the shaft is long, however, the bending tends to increase.　　However, if the total length of the shaft is long, bending tends to increase, so a larger allowance should be considered.　　Even if you have gone to the trouble of creating a hardened layer, it would be a complete disaster if there is not enough room for polishing and the core comes out, or if, on the other hand, the hardened layer is thinned out due to excessive grinding.

Table 4: Guidelines for accuracy control in linear shaft manufacturing

What is high-frequency quenching to achieve results?

Below is a summary of the notes.

• Induction hardening is a technique to harden only the surface in a short time by induction heating.
• High energy efficiency and short delivery time due to self-heating by electromagnetic induction
• Flexible control of hardening layer depth by using different frequencies
• S45C is ideal for general-purpose shafts and SUJ2 has extremely high wear resistance
• Use of a tempering material as a pretreatment is desirable to stabilize heat treatment quality
• The effective hardened layer depth shall be specified on the drawings as the distance to the limit hardness.
• Select appropriate limit hardness according to carbon content based on JIS G 0559
• The property of maintaining core toughness contributes to longer life of sliding parts subject to impact.
• Drawing instructions strictly specify processing details using JIS symbols such as HQI-HT
• Design on the assumption that bending occurs due to thermal stress and transformation stress
• Straightness assurance by the straightening process is the key to supporting precise slide movements.
• Polishing allowance of 0.3mm to 0.5mm or more should be secured in consideration of deformation after heat treatment.
• Center holes at both ends greatly improve heating and polishing accuracy
• Threaded and stepped parts are indicated outside the hardening range to prevent cracking problems.

That's it.