Heat Treatment of Mould

Chan Ka Man Carmen
Feb 2001

Introduction

The Principles of Heat Treatment

The Heat Treatment Process

Quench Media

Surface Hardening

Introduction

The term heat-treatment embraces many processes employing combinations of heating and cooling operations, applied to moulds and dies, tools and machine components so as to produce desired mechanical properties, with attendant characteristics related to particular types of 'in-service' applications. Steel is the most common metal being treated.

It accounts for more than 80% of all metals. The various processes may be broadly classified as:

• Hardening process -This process is intended to produce through hardened structure by quench-hardening. This is the most common heat treatment practice for mould making industry in Hong Kong. Hardening increases wear resistance and strength of material and provides toughness after Tempering, so it is widely used in Hong Kong for increasing the life of moulds as well as mechanical parts of machinery.However, hardening often results in turning the structure of the work brittle. Besides, internal stress increases tremendously while machinability and ductility of the metal decrease. 

• Softening processes - These processes are intended primarily to soften the material, such as Annealing; also those intended primarily to remove stresses either inherent or consequent upon prior operations, but generally resulting in a softer structure. The latter processes include stress relieving and process annealing.

• Toughening process- This process is intended to produce a structure possesses good strength and ductility in steels by means of Normalizing. Improved machinability, grain structure refinement, homogenization and modification of residual stresses are among the reasons for which normalizing is done.

• Case-hardening process - This process is employed to produce a 'case' or surface layer substantially harder than the interior or core of the workpiece. They include carburizing, nitriding and induction hardening.

The Principles of Heat Treatment

Summary

1. Heat treatment consists of Heating-Up and Cooling-Down process.
2. Heating up the steel will change the microstructure to Austenite.
3. Cooling down the steel at different cooling rates will change the microstructure from Austenite to different structures correspondingly.
4. Change in Microstructures result in change in mechanical properties.
5. By heat treatment, we can change the mechanical properties of moulds and machine components to our desired state.
6. For example, to harden the mould will increase strength and wear resistance resulting in longer mould life.
7. To anneal a hard steel bar will soften it to a state good for machining.
8. To normalize a steel bar will toughen it to a state good for impact.

Change in Micro-structure

If a steel bar is heated, it is found that at a specific temperature, which differs with each class of steel, important structural alterations begin to take place in it. This specific temperature marking the beginning of the structural change is known as the 'lower critical temperature'. This change concern the composition of the steel, that is soft ferrite iron and a hard, brittle substance called Cementite (Fe3C). The lower critical temperature is the point at which ferrite begins to transform to another structure called 'austenite' and iron carbide starts to dissolve in the 'austenite'. Eventually, if the temperature is raised high enough to a temperature known as the upper critical temperature, all the steel will become austenite.

Iron-carbon equilibrium diagram

Simulation on changes in micro-structure

The following diagrams show the changes on micro-structures under various heat treatments. You may choose the cooling temperature, cooling rate and step up the change by selecting a higher simulation rate.

Quench-Hardening

If a steel bar is rapidly cooled from it's upper critical temperature by plunging it into a coolant such as water or oil (termed quenching), the effect is to transform the austenite into a structure called 'martensite'. Martensite is a very hard, but brittle constituent of steel.

For steels having a carbon content less than 0.8%, a temperature at between 30 deg C to 50 deg C above the upper critical temperature (723 deg C) is used for quenching and the resulting structure comprises martensite and ferrite.

For steels having a carbon content more than 0.8%, a temperature at between 30 deg C to 50 deg C above the lower critical temperature (723 deg C) is used for quenching and the resulting structure comprises martensite and cementite (iron carbide).

Martensite is hard and high in tensile strength. However, the exceedingly brittleness and very low toughness as well as its intrinsic 'high' internal stresses' render it unsuitable for any form of application. In order to attain the desirable combination of strength, hardness and ductility it is necessary to reheat the steel to a predetermined temperature at below the lower critical temperature. This re-heating after quenching is called tempering.

Time-temperature transformation (TTT) diagrams

A Continuous cooling transformation (CCT) curve compared to TTT diagram.

Tempering

During tempering, martensite undergoes a transformation process from that of carbon atoms supersaturated in iron to a structure termed tempered martensite which consists of highly dispersed submicroscopic carbide particles in a ferrite matrix. The extent of increase in ductility hence toughness and the corresponding reduction in hardness and strength is a function of tempering temperature and time.

Annealing

If a steel bar is cooled slowly in a furnace from a temperature above its upper critical temperature to a temperature below the lower critical temperature, the structure of the steel will become ferrite and cementite again. This steel consists of a somewhat coarser grain structure that is low in strength, high in ductile and soft. This process of heat treatment is called 'Annealing'.

Normalizing

However, instead of cooling in furnace as described above, the steel is taken out from the furnace and cooled in still air, it is termed 'Normalizing'. A normalized steel bar possesses higher strength and toughness than its annealed counterpart.

1045 Steel under Various Heat Treatments

The Heat Treatment Process

Basically, Heat Treatment just consists of heating up and cooling down process. These process can be further divided into four steps.

Quench media vary in their effectiveness, and one can best understand the variation by considering the three stages of quenching:

1. At the first stage, hot metal vaporizes and forms a gaseous layer between the metal and the liquid. Cooling is slow through this vapor jacket.
2. At the second stage, large quantities of heat removed by the vaporization mechanism. As a result, metal is cooled rapidly.
3. At the third stage, metal cools to below the boiling point of quenchant, heat transfer takes place by conduction across the solid-liquid interface.

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Quench media vary in their effectiveness, and one can best understand the variation by considering the three stages of quenching. When a piece of hot metal is first inserted into a tank of liquid quenchant, the liquid adjacent to the metal vaporizes and forms a gaseous layer between the metal and the liquid. Cooling is slow through this vapor jacket (first stage) since all heat transport must now be through a gas. This stage will occur if the temperature of the metal is above the boiling point of the quenchant. Soon bubbles nucleate and break the jacket; liquid again contacts the metal, vaporizes (removing its heat of vaporization from the metal), and forms another bubble; and as the bubbles are removed, the process continues. This second stage of quenching provides very rapid cooling as a result of the large quantities of heat removed by the vaporization mechanism. When the metal cools to below the boiling point of the quenchant, vaporization mechanism. When the metal cools to below the boiling point of the quenchant, vaporization can no longer occur. Heat transfer must now take place by conduction across the solid-liquid interface, aided by convection or stirring within the liquid. This is the third stage of quenching.

Types of Quenchant

Many types of fluid have been used for quenching including water, mineral and animal oils, molten salts and metals, and organic polymer solutions. The three most widely used quenching media are:-

1. Water
2. Mineral oil based products
3. Synthetic polymer quenchants, a new technology in quenching

To obtain desired hardness, the choice of cooling medium is often just as important as the choice of steel. A successful final result is due in large measure to the mutual interplay between the above two factors. The following graphs show the different cooling curves and cooling rate of various quenching media. When these graphs are used together with the TTT and CCT diagram. The most desirable quenching condition can be found.

Cooling curves for various quenching media, derived by means of a silver ball.

Cooling rate of various quenching media tested by means of a silver ballOIl

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Water

Water is a fairly good quenching medium because of its high heat of vaporization and the fact that the second stage of quenching extends down to 212&degF (100&degC), usually well into the martensite range or even below. Water is also cheap, readily available, easily stored, nontoxic, nonflammable, smokeless, and easy to filter and pump. However, with a water quench, the clinging tendency of the bubbles may cause soft spots on the metal. Agitation is recommended when using a water quench. Still other problems associated with a water quench include its oxidizing nature, its corrosiveness, and the tendency for excessive distortion and possible cracking.

Brine

Brine (salt water) is a more severe quench medium than water because the salt nucleates bubbles, forcing a more rapid transition through the vapor jacket stage. Unfortunately, brine also tends to accelerate corrosion problems unless it is removed completely after the quench Different types of salts can be used, including sodium or potassium hydroxide, and various degrees of agitation or spraying can be used to adjust the effectiveness of the quench.

Oil

When a slower cooling rate is desired, oil quenches can be employed. Various oils are available that have high flash points and different degrees of quenching effectiveness. Since the boiling points are often quite high, the transition to third-stage cooling usually precedes the martensite start temperature. The slower cooling through the Ms-to-Mf transition leads to a milder temperature gradient within the piece and reduced likelihood of cracking. Problems associated with oil quenchants include water contamination, smoke, fumes, spill and disposal problems, and fire hazard. In addition, quench oils tend to be somewhat expensive.

Polymer Quench Solutions

Quite often, there is a need for a quenchant that will cool more rapidly than the oils, but slower than water or brine. To fill this gap, a number of polymer quench solutions (also called synthetic quenchants) have been developed. Tailored quenchants can be produced by varying the concentrations of the components (such as glycol polymer and water). The polymer quenchants provide extremely uniform and reproducible results and are less corrosive than water and brine and less of a fire hazard than oils (no fires, fumes, smoke, or need for air-pollution-control apparatus). In addition, they tend to minimize distortion. Many of the polymer quench mediums, however, are extremely sensitive to concentration changes and reauire constant monitoring and control during use.

Salt Bath

When slow cooling is desired, molten salt baths can be employed to provide a medium where the quench goes directly to the third stage of cooling. Still slower cooling can be obtained by cooling in still air, burying the metal in sand, or a variety of other methods.

Oil emulsion

By emulsifying water and "water-soluble oil in various proportions it is possible to obtain cooling media of various cooling capacities. In this respect, however, such media might be inferior to the oil itself. If water is inadvertently added to ordinary quenching oil this may lead to cracking on hardening particularly if the steel is a deep-hardening one because then the martensite formation will start at the surface considerably in advance of the center which, when it transforms, will increase the stresses in the surface. If the water and oil are not properly emulsified, the former having collected at the bottom of the tank instead, a rapid heating-up of water followed by steam generation may give rise to an explosion.

 

The presence of water may be ascertained by dipping a narrow-bore tube, preferably of glass and temporarily closed at its upper end, into the tank at its lowest point. On opening the tube the operator lets the quenching fluid rise in the tube. The opening the tube the operator lets the quenching fluid rise in tube. The opening is then again closed, the tube withdrawn and its contents examined.