Carburised parts are generally heat treated for following reasons:
1. To develop hard and wear resistant surfaces.
2. The long lime and high temperatures during carburising may cause grain coarsening in core and or case. These may be refined to improve their properties like impact strength, etc.
3. If carburising has resulted in cementite network at the surface, it induces brittleness to cause cracks even during grinding. It has to be broken, if possible.
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To improve the wear resistance and hardness, the carburised components are quenched from the austenitic range. The component may cither be quenched from the carburising temperature, or may be reheated to appropriate austenitising temperature and quenched. The direct quenching is less time consuming and cheap, but is not possible in pack carburising practice. Thus, pack- carburised parts are invariably reheated and quenched.
In many cases such as given below and more, even the gas and liquid carburised parts are also reheated and quenched:
1. If a carburised part needs some machining before hardening, then it is cooled slowly to be in soft state so that machining can be done.
2. Coarse grains developed during carburising can be partly refined by reheating to minimum austenitising temperature for hardening.
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3. Higher surface carbon content on quenching results in having higher retained austenite there with simultaneous decreased hardness. Grinding these parts may produce cracks. Generally, less than 15% retained austenite may be tolerated depending on the application when the carburised part is reheated for quenching, some diffusion of carbon takes place which moderates the carbon content at the surface. On quenching now, the amount of retained austenite becomes less at the surface.
Fig. 8.28 illustrates the cross-section of a carburised and slowly cooled steel. The surface layers consist of hypereutectoid structure and, with increasing depth, the carbon content decreases and thus, shows 100% eutectoid structure, which changes to hypo-eutectoid structure, and ultimately the hypoeutectoid structure of the core is obtained.
Effectively a carburised steel is a combination of steels containing different amounts of carbon. Thus, the heat treatment of a carburised steel is complicated because, in the same piece, the carbon content varies. Still, heat treatment can be given to obtain desired results based on metallurgical principles learnt so far.
The heat treatment of the carburised steel depends on:
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1. Nature of steel- Inherently fine or coarse grained steel.
2. Carburising temperature.
3. Composition of the case and of the core.
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4. Expected properties.
5. Method of carburising.
The simplest way to understand the heat treatments after carburising is to classify the processes in two categories based on the carburising temperature used and the developed carbon content of the case:
1. Heat treatment to steels which have been carburised at temperatures only slightly above the AC3 temperature of the core, for example T1 in Fig. 8.29.
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2. Heat treatment to steels which have been carburised at much higher temperature than Ac3 temperature of the core, such as T2 in Fig. 8.29.
Each of these categories can be further subdivided into three classes depending on the maximum carbon content of the case (see Fig. 8.29).
Class X- are heat treatments to steels whose carbon content of carburised case is up to eutectoid carbon.
Class Y- are heat treatments to steels where carburised ease is of hypereutectoid carbon content but the carburising temperature was higher than the Acm temperature of case.
Class Z- are heat treatment to steels whose carburised case is of hypereutectoid carbon content but the carburising temperature was lower than the Acm temperature of the case:
Category 1 (X):
Carburising temperature is slightly above Ac3 and the carbon content of the case is up to eutectoid carbon (0.77%). This is the simplest case and one of the two heat treatments may be used. The first treatment consists of quenching the steel directly from the carburising furnace.
This is the cheapest and simplest method. The second method is superior to the first method as it results in better properties of the case. For the carbon content X of the case, its austenitising temperature for hardening (slightly above Ac3) is T3.
This means that at the end of the carburising, the case has been overheated to temperature T1 (by temperature T1 – T3 °C). Thus, the case may not be as line grained as desired. To obtain this, (he steel is cooled slowly after carburising and reheated to temperature T3 (Fig. 8.29) to produce fine austenite grains in case. It is quenched and tempered. Invariably, in carburising, the case is hypereutectoid.
Category 1 (Y):
This is the most common state of carburised steel. Three heat treatments can be given. The cheapest and the simplest method are to quench directly from the carburising temperature.
The hardened case shall have less hardness because; of large amount of retained austenite obtained and the absence of proeutectoid cementite (cementite is slightly more hard than martensite. Presence of proeutectoid cementite is desirable but not in the form of network along the grain boundaries).
Moreover, if the steel is inherently coarse grained type, the coarse grains result in lower toughness. Thus, direct quenching is not able to induce best properties. The second method consists of slow cooling to room temperature and then reheating to above Acm temperature, quenching from there and then tempering. Acm is a steep line.
Hyper steel heated to above Acm temperature invariably suffers from grain coarsening. Thus, by second method, grain refinement of core takes place, but the case shall not be any tougher and harder than in the first method. To induce the best properties, the commonly used method called “double heat treatment” is used. It is a bit expensive.
Double Heat Treatment:
Slowly cooled carburised steel is heated to above Ac3/Acm (whichever is higher) temperature (grain size of core is refined and cementite network is dissolved and shall be eliminated), and then quenched.
The steel is then heated to above Ac1 temperature, quenched and tempered. This heating refines the case (no overheating of case occurs) and induces high hardness in the case. Double heat treatment gives the best combination of a hard, wear resistant, fine-grained case, and a tough, refined core. This method may give greater dimensional changes. Schematic diagram is shown in Fig. 8.30.
Category 1 (Z):
If carburising is done under high carbon potential of atmosphere and for a long time, then at the carburising temperature, the carbon of the case is high enough to have a structure of austenite and cementite (proeutectoid) i.e., cementite network is present at the austenite grain boundaries.
Three heat treatment cycles are possible. The simplest and cheapest is to directly quench from the carburising temperature (and temper). The cementite network is preserved in this method which makes the case highly brittle-undesirable.
The network can be eliminated in second method, which consists of first cooling and then reheating to above Acm temperature of the case, quenching and tempering. The steel shall not be tough because grain coarsening occurs of core (Acm temperature is much higher than Ac3 temperature of the core see Fig. 8.29) and case (heating above Acm causes it).
In the third method, after the first quench as above, it is heated again to above Ac3 temperature, quenched and tempered. Though case is refined here, but core remains in coarse-grained condition. Properties of case may be more important in quite a few applications.
Category 2:
As the carburising temperature is much higher than the Ac3 temperature of the core, the core is -invariably coarse-grained. Even the case here shall have coarse grains. Double-heat treatment is normally recommended in all cases.
Category 2 (X):
If refinement of the case only is desired, then the slowly cooled steel is heated to above Ac3 temperature of the case (fine grains of austenite in case are obtained), quenched and tempered. Double heat treatment consists of first heating the slowly cooled carburised steel to above Ac3 of core (to refine it), cool, and reheat to above Ac3 temperature of the case, quench and temper.
Category 2 (Y):
This case is similar to 1(Y). The best results are obtained by double heat treatment.
Category 2 (Z):
It is very difficult to get satisfactory results in such cases. The carburised case at the carburising temperature has network of cementite along grain boundaries of austenite. If quenched, then the case is highly brittle-very poor case. The slowly cooled carburised steel if heated to slightly above Ac3 temperature, of the core (to get refined core), the case has cementite network along austenite grain boundaries.
If quenched, then highly brittle case results in impairing toughness of the steel. If steel is attempted (see Fig. 8.29) to dissolve cementite network by heating it, burning and fusion takes place. The state of steel in 2 (Z) is quite hopeless.
Tempering:
Carburised parts too are tempered after hardening at 160- 190°C. This helps in increasing toughness of the case, reducing its susceptibility to crack on grinding without decreasing the hardness much.
Sub-Zero Treatment (Cold Treatment):
Sub-zero treatment is often applied to hardened carburised components, specially of those alloy steels which retain large amount of austenite. This not only improves hardness and wear resistance but also gives dimensional stability to parts. Sub-zero treatment is done immediately after quenching before stabilisation of austenite occurs.
Table 8.7 gives effect of sub- zero treatment. It is evident that the first cycle is more effective. The major increase in hardness takes place at or near the surface, where the carbon is maximum and also the retained austenite.
Now a days with the easy availability of a bit expensive Al- killed steels which allow the use of reasonable high carburising temperatures without the danger of excessive grain growth, simple carburising cycles as given in Fig. 8.31 are used. In great majority of applications, cycle no. 1 is used, and often without a tempering operation. In all cases, good carburising practice and heat treatment cycle requires that the maximum carbon content of the case should be at the surface of the steel.
During hardening the carburised steel, austenite to martensite transformation begins at the boundary of case and the core, and progresses outward towards the surface of the steel. Large volume expansion occurs in the case due to this transformation, which does not occur in core. This puts the surface layers in compression, which is beneficial to the component to bear higher tensile stresses along-with better fatigue properties.
But if the carbon gradient in the case takes the shape as in Fig. 8.19, i.e. maximum carbon is not on the surface of the component that means, surface layers are not the last to transform to martensite these layers are then put in tension, which is not desirable. 
Grange’s method to obtain maximum hardness of 800-900 VPN in the case in shorter carburising time resulting in fine grains in the case as well as in core and applicable to cheaper Si-killed steels where toughening is obtained by grain refinement. Thus, even Ni-free steels can be used as shown in Fig. 8.32 Carburise at (1035°C) around 115°C more than usual temperature for short time.
Step II tempering at 370°C changes retained austenite to bainite (lower). Rapid austenitisation in step III needs induction heating, probably and then oil quench. Step IV is tempering in 175- 225°C. Fig. 8.33 compares the properties with a conventional method.
Practical Thumb Rules:
To attain a minimum hardness of 60 HRC after quenching, some practical tips above heat treatments are:
(i) For Pack Carburising at 900-925°C:
(ii) For Liquid Carburising at 850-900°C:
(iii) For Gas Carburising:
Gas carburising temperature of 900-940°C, is used to obtain required case depth with a surface carbon of 0.7-0.8%. Then temperature of furnace (or parts) is decreased to 830°C to then quench in suitable quenching medium.
The choice of case depth is a difficult, experienced based job. In case-hardened gears, the design load is proportional to gear module. The depth of case hardening is usually kept as 0.15-0.18 times the module. Case depths of more than 2.0 mm are rarely used in gears.