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Why all the questions?

06 April 2018

“The services we offer will give a component the highest thermal shock that it will receive in its lifetime,” states Roger Haw, managing director of Flame Hardeners Ltd.

A choice of heating and quenching techniques is available to help customers achieve their requirements. However, the two most important factors are restricting the heat to the area that requires selectively hardening and applying quench with a cooling rate suitable for the specific material.

“Heat treatment contractors accept parts from customers, varying in value from fractions of a penny each to over £100,000 each. The parts are issued free to the contractors who are then expected to employ the best of their knowledge and expertise to give the customer a part to his specification,” explains Haw.

Modern heat treatment techniques and the acquired knowledge of their applications lead to increasingly more outsourcing. However, before they can develop process routes etc., contractors require some basic information about the part as received.

Roger Haw outlines some of the common questions asked by heat treatment contractors - and the reasons why they ask them.

What is the material?

We need to know what the material is in order to give it the correct treatment in order to provide the hardness and depth required without cracking the component.

Flame and induction hardening involves instant heating to temperatures in excess of 800°C, followed by very rapid quenching. This produces very high volume rates of change, giving faster cooling on the surface and slower cooling sub surface. Such changes can set up a stress system which can lead to fracture or cracking, sometimes after quenching, or in the early stages of tempering. To avoid such undesirable effects, we employ a variety of quenching mediums to control the rate of cooling. Different rates of cooling are required for different materials – e.g. carbon steels, alloy steels, cast iron, tool steels.

Better still, a certificate of chemical analysis helps us offer the best treatment possible.

What hardness is required?

Tempering is a process which is time and temperature dependent, and we need to know what temperature to apply and how long to apply it in order to meet your specification.

When heated and quenched, all steels will reach a hardness level compatible with the chemical analysis of the material. It is usual practice to temper back from this level to the level specified for any particular applications. 

The hardness level you require may not be achievable using the material specified. In such instances it is our responsibility to advise you of the probability results and, in some cases, suggest alternative materials.

What depth of hardening is required?

The process we develop will produce the controlled depths of hardening. 

The depth of hardening produced by induction hardening techniques is dependent on the frequency used. The higher the frequency, the lower the depth of hardening. A frequency of above 200KHz will give a depth of 0.015” to 0.100”, 10KHz up to 0.20”, and 3KHz up to 0.8”. Flame hardening will give depth of 0.10” to 1.0”, depending upon the technique used.

The depth of hardening is also affected by the choice of material – an alloy steel will generally have higher hardenability than a plain carbon steel.

Where does the item require hardening?

The advantage of the flame and induction hardening process is that the heat and quench is applied only to those areas of the component requiring improved strength or wear resistance. This means that the energy used is less than that produced by other heat treatment processes.

Your drawing must indicate which areas are to be hardened.

Where is the drawing?

We need a drawing in order to ensure that the component has been hardened to your requirement or the requirement of the ultimate customer. 

We are often asked to harden parts without a drawing being provided, or we are presented with part drawings. Even where drawings are supplied, they often refer to the ultimate customer’s own specification.

We cannot guarantee that a component satisfies the specification if we have not seen it. This is particularly relevant to components for the oil and gas industries, and other industries where parts are safety critical.

What is the previous metallurgical history of the component?

Previous treatments, such as rolling, forming, and stress relieving can all play a part in causing distortion during hardening.

We could potentially be adding to or relieving previous residual stresses.

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