Hardening and Tempering with Induction

What is Hardening?

Hardening can be defined simply as any process by which the hardness of a material is increased and ductility is decreased. This process is employed to reinforce surfaces prone to high wear and enhance the overall longevity of parts. While various hardening methods exist, each with its suitability depending on the material, Induction technology is frequently applied in a specific hardening process known as Quenching and Tempering.

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What is Quenching & Tempering?

Quenching and Tempering constitute a hardening process exclusively applicable to medium to high-carbon steel. Initially, the steel undergoes heating to a sufficiently high temperature, causing a transformation in the crystal structure from ferrite to austenite. In this altered state, the steel can absorb a higher amount of carbon than under normal circumstances. Subsequently, the steel is rapidly quenched using water, oil, or a water-polymer solution in induction equipment. This rapid cooling prevents the formation of carbon precipitates that could compromise the final hardness of the steel.

As the steel cools to a lower temperature, it attempts to revert to its original low-temperature state. However, having absorbed more carbon than its original state can accommodate, the steel transforms into a distinct crystal structure known as martensite. Martensite is exceptionally hard due to its distorted crystal structures and elevated carbon saturation. While the steel achieves high hardness at this point, it also becomes quite brittle.

Tempering is the subsequent step, involving reheating the steel to a lower temperature to slightly reduce hardness and, consequently, brittleness. The specific temperature for tempering depends on the desired hardness level. Once the target hardness is attained, the steel undergoes another quenching process to prevent any residual heat from further tempering the steel.

Quenching & Tempering with Induction

Induction technology is predominantly employed in Quenching and Tempering procedures, where it offers notable advantages. Achieving the desired hardness profile in this procedure demands exceptional precision in both the heating and quenching of the steel. Even slight variations, such as heating for a moment too long or quenching at an incorrect temperature, can lead to significant disparities from one part to another. Hence, maintaining precise control over the process becomes crucial, and induction technology stands out by providing superior control compared to other heating methods.

The automatic handling and fixturing of components for heating and quenching contribute to high production rates and consistent results across various parts. Induction stands out as the fastest method for hardening and heat treating, resulting in minimal distortion, no surface decarburization, a fine-grain microstructure, and meticulously controlled hardness patterns. For any facility aiming to elevate production to the next level, induction emerges as the ideal solution.

Induction Case Hardening

Selective induction case hardening can enhance the performance of parts by providing hybrid mechanical properties, hardness where needed on wear surfaces, and ductility in the core to provide impact resistance.

Selective induction hardening has the ability to specifically target and apply heat rapidly to a localized area of a part. As a result, the part develops a layer, or case, of hardened material. This is ideal for parts that are highly stressed in operation and require a combination of mechanical properties. For example, high yield strength, fatigue resistance, and resistance to wear at the same time.

The precise hardness pattern can be controlled by appropriate adjustment of the frequency used, induction coil geometry, power level, and the location of the part in the coil. The hardness pattern remains highly consistent from part to part due to industry-leading precision in Radyne systems. Rotation during heating assures a uniform case.

Progressive Induction Hardening | Induction Scanning

Frequently, parts require surface hardening in selected areas to provide optimum performance and long service. Induction hardening, using progressive heating and quenching, provides an economical way to process shafts or other parts.

By progressively passing a steel shaft through a heating coil into a water quench, the outer skin can be heated, quenched, and hardened without affecting its core. When a completely uniform case is required it is often necessary to rotate the shaft. The coil and the associate water quench usually form one unit, as the position of the water quench with respect to the inductor is very important. The water supply is often fed through the coil itself, as illustrated. Controlled scanning of shafts through the induction coil and quench ring while rotating produces controlled case depths over adjustable lengths of the shafts, all in on an automated cycle.

Additional reading:
Understanding AC Condensing Units in Facilities Management

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Control of the case depth depends upon the power input from the generator and the downward speed of the shaft through the induction coil while the length of the hardened section is controlled by a pyrometer which is set to suit specifications. Once the desired case depth and hardness patterns are found by matching equipment parameters, these settings can be made consistent for a repeatable process across every part.

While the application described involves surface hardening, it is adaptable for other heat-treating operations that require progressive heating, such as annealing and tempering.

chris_seven

Registered

Join Date: Feb

Location: UK

Posts: 2,230

Cranks

Chaps,

I am always surprised about the variety of opinion on the subject of 911 cranks and their heat treatment and the variety of solutions that seem to be advised.

I have put up a few posts on the subject but it doesn't hurt to repeat some of these comments.

I believe that all early cranks were Tenifer treated which is the Trade name of a process known as ferritic nitrocarburising.

The same process is sold in the UK as Tuftriding and in the USA is also known as Melonite.

There may be some small differences in chemistry but the basics are all the same.

This process was typically carried out in a cyanide bath at around 570 degC.

This process also known as Salt Bath or Soft Nitriding and is certainly the poor relation of the genuine Nitriding Process.

As has been stated the surface layer of a tuftrided part is very thin being only a few thou so the influence on fatigue life is modest.

There are many claims that Tuftriding can add between 20 and 60% to the fatigue life of a component but this statemnt does show a lack of understanding of the nature of the fatigue process.

The idea of Nitriding is to introduce surface residual stresses that inhibit the initiation of short fatigue cracks and thus significantly increse the fatigue endurance limit of the component.

An increase in life of 20% assumes that the component will fail at some time and cranks need to be designed for an infinite fatigue life. The fact that they do fail in fatige is due to the stoichastic nature of the fatige process or some other manufacturing defect such as too sharp a radius, as has already been mentioned.

Nitriding has a much greater and substantial impact on fatigue life than tuftriding will ever manage.

The scuff resistance offered by both processes is useful but is not at the heart of the process.

I have to say that without deatiled knowledge of the steel used for 911 cranks I wouldn't nitride. Nitriding generally requires a specialised steel with EN40B being a common example but 722M24 and M897M39 are good alternatives. Aerospace materials such as S132 would also be good.

Nitriding non-nitriding steels can lead to brittle surfaces which can spall and flake in a very unhelpful manner.

I am also concerned to hear that nitriding can cause distortion and bending.

Traditional nitriding in a gaseous environment would almost always be carried out with the component being hung and therefore in tension.

A typical heat treatment/manufacturing cycle for a nitrided part would be to rough machine - often from a forged blank, harden and temper to say a T condition, and then machine to a reasonably close tolerance.

I would then excpect the part to be stabalised at 590 degC to allow stress relief and any movement to occur.

the part could then be finish machined and Nitrided at 570dgC with no further distortion.

The white layer produced during nitriding would then be removed form the journal surfaces by grinding and this allowance would be made prior to heat treatment.

I am intrigued to find that cranks are now being induction hardened as a means of improving fatigue life and I would like to see more detail of how this is carried out as i would be concerned that the steels needed may be OK for normal road engine but coul be limited for high rpm race motors.

Certainly a great many road cars now use an Austenitic Spheroidal Graphite cast Iron which has excellent strength ductility and fatigue life.

This material was developed at BCIRA in the UK and I supplied them a couple of fatigue testing machines more years ago than I like to admit.

I would also like to answer a few of the other questions that cropped up in this thread, I know they are old but it may shed a little light.

Flame hardening is a very old technique which was used to surface harden relatively high carbon steels.

The composition of the flame is irrelevant - this idea is to heat the steel very locally and then obtain a queching efffect due to the coduction of heat waya from the surface into the body of the steel. This technique has been replaced by induction hardening which uses an AC field and the 'skin effect' to locally heat the metals surface instead of a flame. The improvenemt in fatigue life offered by these process would IMHO be very limited.

Annealing is a very straightforwad process. All Ferritic Steels are at their most simple an Iron/Carbon Alloy. Iron is only soluble at room temperature to around 0.02%. Whena steel writh more carbon than this cools it form a structure that comprises of iron and iron carbides. The morphology that results depends on the carbon content and the way it was cooled. Steels which have been hot rolled on a mill are often cooled quite quickly and be quite hard, steels which have been cold worked also harden. At around 723 degC steel will begin to change crystal structure from Body Centred Cubic to Face Centred Cubic and the solubility of carbon increases significantly. Depending on carbon content this process will be complete ast 960degC and a new single phase structure known as Austenite will be present. If the material is then slowly cooled, normally by the simple expedient of turning off the furnace and not removing the material until it has reached room temperature, the material will be in its softest condition. ie Annealed.

Quenching takes place by heating the steel into the Austenite range and then rapidly cooling. As the steel tries to change its crsytal structure from FCC to BCC the carbon atoms which have migrated in the crystal lattice prevent this from happening as the tme domain available locks tham into plave (Fick's Second Law of Diffusion and all that) and a very heavily distorted structure results. This is known as a Martensite Reaction.

To give a basic idea a file is a 1% carbon steel which has been fully quenched and not tempered.

Last edited by chris_seven; 07-19- at

10:12 AM

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