Hardening of Steels: 3 Defects | Metallurgy

The main defects produced during hardening are: 1. Mechanical Properties not up to the Specifications. 2. Soft Spots. 3. Quench Cracks.

Defect # 1. Mechanical Properties not up to the Specifications:

Hardness is the most common test used for checking hardened tools and components, but other tests are also done on machine components to check the other mechanical properties to confirm to specifications. As a hardened component is always immediately tempered, these quality assurance tests are usually done on tempered parts, but often the reason of unsatisfactory hardness, or mechanical properties are found to originate during hardening. The most common defect in hardened tools or components is too low a hardness.

Such a defect might arise due to following factors:

(i) Insufficient Fast Cooling:

The coolant might have been overheated, or even polluted. The presence of scale, oil, etc. on the surface of steel also provides slower cooling rate. Inadequate agitation, or circulation of coolant may also result in this. In the method “through water to oil,” the component might have been transferred after short duration of time in water. In the method, interrupted quenching, time of holding after withdrawing from water, might have been more now.

(ii) Shorter Austenitising Time:

If instead of fine lamellar pearlite, the new stock of steel may have globular pearlite in microstructure. As globular pearlite takes much longer time to form homogeneous austenite for hardening, the earlier cycle does not give hardness of required specification. Insufficient dissolution of carbides in austenite results in low hardness.

(iii) Lower Austenitising Temperature:

The reasons could be a few. The furnace is not designed well so that it has non-uniform temperature, and some parts were heated to lower temperatures; considerable drops of temperature might occur if the distance between furnace and cooling bath is large, so that transfer of parts took longer time than visualised; the temperature control unit is not proper, or needs calibration with the passage of time; the composition of new material could be different, which requires higher austenitising temperature.

(iv) Decarburisation of Surface:

The furnace atmosphere may be more decarburising, or it is not being controlled properly, or the part is heated for too long a period; if decarburisation occurs, the hardness of martensite dependent mainly on the carbon content, drops.

(v) If too high temperature is used, more retained austenite results in low hardness.

Defect # 2. Soft Spots:

A hardened component has non-uniform distribution of hardness, i.e., the surface is found to have hard as well soft areas. The soft areas are called “soft spots”.

The reasons could be:

(i) Some areas of the surface might have got decarburised, which give soft spots.

(ii) Scale etc. adhering to the surface may cool these areas slowly to result in soft spots.

(iii) Temperature of the coolant is too high to extend vapour-blanket stage, resulting in lower hardness.

(iv) Tools have not been agitated vigorously to result in uneven cooling, and so non-uniform hardness results.

(v) With insufficiently vigorous, or irregular agitation of the tool in the cooling tank, the tendency to form vapour-blanket on some surfaces increase (Fig. 6.18), where the cooling rate drops to result in soft spots. Water spraying eliminates soft spots completely.

(vi) If a large number of parts lying over one another are quenched at a lime, or if there is insufficient coolant, it can lead to soft spots.

(vii) Oil, if used as coolant, and if, has high viscosity (temperature of the oil is less) may result in soft spots.

(viii) A rough surface keeps the vapour intact to cause soft spots.

The reason of soft spot in each case must be analysed and suitable measures must be taken to avoid it.

Defect # 3. Quench Cracks:

Quench cracks occur as a result of tensile stress, whose magnitude becomes higher than tensile strength of the steel. Steels having lower M_s – M_f temperatures, i.e., high carbon steels and steels containing alloying elements, with enough carbon to produce brittle martensite are more prone to quench cracking. Fig. 6.19 illustrates simple illustration of quench cracking by residual stresses during quenching.

Some of the most important reasons for quench cracks are as follows:

(i) More Time Lags between Quenching and Tempering:

A good thumb rule is to transfer the quenched part from the coolant to tempering furnace is, when one can just hold it in hands in the coolant (it is warm with temperature 60 – 80°C).

(ii) Overheating the Steels:

For some reasons, if overheating occurs, it leads to grain growth of austenite which results in coarse martensite of high brittleness and more retained austenite, which ultimately causes brittleness-resulting in cracks.

(iii) Use of Wrong Coolant:

A coolant which provides unwanted faster rate of cooling leads to quench cracks. An alloy steel which can be hardened in oil with good results, if had been hardened in water, or brine, might have resulted in quench cracks.

(iv) Faulty Design of a Component:

A part should be designed as balanced as possible avoiding as far as possible:

(a) Heavy sections adjacent to a thin section.

(b) Sharp corners and sharp transitions between sections.

(v) Improper Selection of the Steel:

A high carbon steel having section greater than 1.27 cm, if quenched in water, may show cracks.

(vi) Grinding Cracks:

Dies, pins, bearing surfaces should be ground to close finish. Grinding uses 30 times more energy than other machining methods and the temperatures rise to 1100°C to 1650°C, resulting in,

(a) Changes in hardness and microstructure,

(b) High stresses to cause surface cracks.

Increased temperature transforms retained austenite to martensite, i.e. increases the stress level further to cause grinding cracks. Presence of large amount of retained austenite increases danger of grinding cracks. If heavy feeds are used for grinding, the temperature of steel surface may become higher than hardening temperature to result in as-quenched martensite, which in next pass, being brittle, causes cracks. Grinding should be done on tempered steels without retained austenite.