Heat Treatment of steels is called the heating and cooling process to achieve certain microstructural features for a wide range of applications. If required properties and microstructural features do not match with criteria than the process is said to be defective. Sometimes all processing steps are carefully followed but properties still not match the criteria. The problem may lie in material design or steel composition leading to defects of heat treatment. For achieving desired properties, it better to select raw material and part design.
Defects of heat treatment which may encounter during the Heat treatment process are;
- Soft Spots
- Lower hardness and strength
- Oxidation and Decarburization
- Formation of Cracks
- Overheating and burning
- Distortion and wrapping
- Temper Embrittlement
Soft Spots (Defects of Heat treatment)
Sometimes, after the steel is quenched from austenitizing temperature, surface hardness varies from region to region. This variation of hardness in quenched steel from point to point is termed as Soft Spots.
The Brinell hardness test is commonly used in industry for large parts that do not have surface finished. Rockwell hardness test is used for finished parts which are considered non-destructive in nature and will not affect surface features of steel parts. For Laboratory testing and specific hardness of phases in steel, the Vicker hardness test is used.
Several reasons which might cause soft spots in steel are;
- Problem with Quenching media: We mentioned in our article, “Effect of Quenching media on Steel” that several media like waters create a vapor blanket stage which reduces the critical cooling rate in a specific area. This results in lower martensitic formation and lower hardness in specific areas leading to defects of heat treatment.
If the temperature of quenching media is high or agitation is improper, this, also, results in the generation of soft spots in steel.
- Localized Decarburization of Steel: Carburizing is, also, one of the major reasons for soft spots. This is often a problem in Pack carburizing if the steel part is not properly packed. This results in uneven carbon distribution resulting in soft cores. Carburizing layer is very thin and excessive grinding may expose soft area which causes soft spots generation.
- Inhomogeneous Microstructures
- Improper handling during Quenching
- Uneven heating: Sometimes steel parts are too large and heavy that, after they placed in a furnace, uneven heating causes weak areas resulting in soft spots in steel.
- Improper Cleaning of Steel part: Cleaning steel before heat treatment is very important. The presence of dirt patches and dust particles can prevent heat diffusion into the steel which causes improper cooling of the steel part. This results in uneven hardness in steel.
Following common practices, you must take to prevent soft spots generation in steel;
- The packing mixture should be properly mixed and packed around the steel part.
- Continuously replace quenching media after set intervals.
- Use coolant during grinding.
- Quenching media should have a properly controlled temperature.
Lower hardness and strength after Quenching
Steel requires martensitic formation for higher hardness and strength. Martensitic steels are commonly used in the defense industry, powder metallurgy, and cutting tools industry. These types of steel undergo a series of heat treatments for achieving desired hardness and strength. After heat treatment, not getting desired hardness or strength can become a cause of stress.
Following are common reasons for lower hardness and stress in steel after heat treatment;
- Lower hardening temperature
For higher hardness and strength, martensitic formation is very important. In case of improper Austenitizing temperature for quenching, there is a possibility that martensitic will not form during quenching. Follow, “Effect of Austenitizing temperature on Quenching of steel”, for understanding the importance of soaking temperature. Complete homogenous austenite should be present at soaking temperature before quenching. It is essential for higher hardness and strength. Alloying elements can considerably affect the austenitic temperature necessary for homogenization. For more information, “Study effect of alloying elements on Steel, Phase diagram and TTT diagram”
- Insufficient Soaking Time
Alloying elements lower diffusion rate and cause slow homogenization at soaking temperature. For achieving higher hardness and strength, complete austenitic formation at soaking temperature is essential as discussed above.
- Delayed Quenching/Slower Cooling Rate
Quenching plays a very important role. A slight decrease in the degree of cooling may cause the start of diffusion-based transformation mentioned in the TTT diagram of steel. This results in Pearlitic formation which has lower hardness as compared to martensite. The cooling rate higher than the critical cooling rate is essential for achieving higher hardness and strength in steel.
- Presence of large amount of retained austenite
We discussed, an increase in carbon can increase the chances of martensitic formation but also lowers the Mf temperature line. With lower Mf, a martensitic transformation from austenite decreases resultantly giving higher retained austenite. Alloying elements also increases the chances of higher retained austenite in a matrix. Tempering is essential for achieving higher hardness in microstructures where retained austenite must be converted.
- Low Hardness in Surface Hardening Treatment
Sometimes, low strength or hardness is observed in carburizing, nitriding or cyaniding treatment. This happens due to improper carburizing material, heat treatment temperature or too much steel in a furnace to achieve homogenous temperature as discussed in Soft spot generation in steel.
Oxidation and Decarburization
During the heat treatment of steel in an open atmosphere, steel may get exposed to environmental gases like oxygen, carbon dioxide, and water vapors. They may react with steel at high temperature and given rise to two important defects of heat treatment in steel;
- Oxidation of steel
- Decarburization of steel
Oxidation of steel
Oxidation of steel takes place in the presence of gases like carbon dioxide, air, and water vapors. A reaction which will take place due to oxidation are;
With the presence of an oxidizing atmosphere, oxide products start developing on the surface of steel above 180oC. The oxide product which forms at lower temperature i.e. 180o C is quite dense and restricts further degradation of steel. That’s why the tempered steel surface can be cleaned and no loss in properties or degradation of the material is observed. When the temperature of steel increases above 450o C, the oxide will become porous and the new surface of the steel will get under attack again and again. This results in material degradation and loss of properties. Oxidation of steel is reversible. The heat treatment atmosphere should be maintained in a manner to minimize the chances of oxidation.
Decarburizing of steel
In certain cases, steel during heat treatment when heated above 650o C, decarburizing or loss of carbon from the surface of steel takes place resulting in loss of mechanical properties like fatigue strength. The defect of this nature is termed as Decarburizing of steel.
If the presence of oxygen and water vapors, the tendency of carbon towards oxygen and vapors increases above 650oC and carbon from the steel surface starts reacting with environmental gases resulting in depletion of carbon from the steel surface. This depletion of carbon from the steel surface is a function of temperature, time and furnace atmosphere.
Following reaction, on steel surface, takes place during decarburizing of steel;
Prevention of Deoxidation and Decarburizing of Steel Surface
- First and foremost, a method to prevent these usual defects of heat treatment is to perform heat treatment of steel in Vacuum or molten salts or a protective atmosphere. The protective atmosphere includes dried producer gas or dissociated products of ammonia.
- If the steel product is not a finished product, then the steel decarburized surface can be removed by grinding.
- Copper plating with a thickness of 0.013 to 0.025 mm can be used for preventing deoxidation and decarburizing of steel.
- Ceramic coatings can be also used for the protective surface of the steel.
- Special surface coatings like borax can also be used for protection.
Quenching is a critical process that involves severe cooling of large structural parts to cause martensitic transformation. This quenching process is always accompanied by several tensile and compressive stresses associated with austenite to martensite transformation. These stresses, in severe cases, result in cracks during heat treatment and counted as defects of heat treatment. Cracks developed during hardening treatment are termed as Quench Cracks. Cracks of these nature are a matter of concern as these cracks make steel render useless and of scrap value.
Quench Cracks Nucleation Process
The linear coefficient of steel is around 11*10^-6. It means when steel is cooled below austenite, contraction of 0.9% will be observable in the material.
Austenite to martensitic transformation results in an expansion of about three times than linear contraction happened during cooling.
This basically means expansion started due to martensitic formation overcomes thermal contraction due to the cooling process. Overall, the expansion of 1.4% takes place in steel during cooling after overcoming the contraction of 0.9%.
With thicker steel castings, where the cooling rate of center and surface largely differentiates, martensitic transformation in center portion gets delayed. That’s why in thick castings when steel is quenched from austenitizing temperature, the surface of steel first converted into martensite.
Surface martensite undergoes the expansion process while the steel core region is still austenite. As surface expansion takes place, it forces the expansion of austenite too. This expansion can easily take place due to the soft nature of austenite. At high temperature, the yield point of austenite is lower which causes plastic deformation to overcome stresses generating in steel due to martensitic formation on the surface.
When the center region wants to undergo expansion as soon as the martensitic transformation in the center takes place, its expansion is restricted by already formed martensite on the surface. Unlike austenite, martensite can not be plastically deformed to release stresses. This expansion of martensite in the center causes large tensile stresses in steel which causes cracks nucleation at the interface of surface and core martensite. This is accounted for as defects of heat treatment.
Cracks nucleated due to thermal stresses within steel during the quenching process are termed as Quench cracks.
Use of Alloying elements for Prevention of Quench Cracks
We discussed above; alloying elements delay the Pearlitic formation thereby shifting c-shape TTT – curve towards right. This lowers the critical cooling rate necessary for martensitic formation. The use of slower heating to reduce the temperature difference between the center and surface of the steel part can prevent a generation of large thermal stresses in the material.
Relieving of Thermal Stresses (defects of heat treatment)
Large and thick castings will always have quenching stresses which will render them unavailable for daily life use leading to defects of heat treatment. It is very important to relieve those internal stresses generated in the material. These internal stresses can be removed by the Tempering steel process.
Tempering steel at a low-temperature range of 250oC can easily remove 80-85% thermal stresses generated during the quenching of steel. Further heating in th
Quench cracks, once nucleated can not be repaired and that’s why prevention of their nucleation is a very important task.
Distortion & Warping
Distortion and Warping of steel is most common problem which can not be completely removed.
Symmetrical change in shape or size of a component is termed as Distortion e.g. contraction in steel component during cooling.
If change is asymmetrical then it termed as warping e.g. thin steel sheets get deformed or lose their straightness during cooling which termed as warping of steel.
With heat treatment of steel, following structural changes will occur during the complete process:
- Steel expansion thermally till heating to AC1 line.
AC1line is crossed, transformation into austenite results in contraction.
- Thermal expansion of austenite upon further heating till homogenization
- Thermal contraction as explained above in quench cracks till Ms line for martensitic transformation or C-shape curve for diffusion-based transformation
- Expansion of steel component during transformation
- After complete transformation, expansion of steel component upon further cooling
- Contraction during the tempering stage
In the case of slow heating and cooling, steel will get enough time to release stresses by bringing change in dimensions. With severe cooling, heat distribution in the sample will not be uniform bringing step-wise changes to cause the development of internal stresses. These distortions can be either residual stresses due to martensitic formation in steel or internal stresses developed due to differential temperature between core and case of steel.
There are two common type of distortions which are observed after heat treatment of steel;
- Size Distortion
This type of distortion takes place due to stages of expansion and contraction in steel during heat treatment.
- Shape Distortion
This type of distortion is manifested due to the bending and twisting of steel commonly termed as warping of steel.
Distortion is a common problem observed during heat treatment of steel and is considered one of the most common defects of steel. The factors which should be optimized to minimize distortion are initial composition, design, initial condition, and machining processes.
Methods to reduce distortion of steel
Methods used for minimization of distortion in steel are as follows;
- Stress Relieving
Manufacturing processes like forging, rolling and fabrication processes like welding have a large number of residual stresses in steel. These residual stresses increase the tendency of steel for shape distortion. It’s in our best interest to perform sub-critical annealing or normalizing to relieve stresses in steel before hardening processes to avoid residual stresses in steel.
- Heating Rate
The fast heating rate may also cause distortion due to differential stresses generated throughout the material as we discussed above. Preheating the steel just below the Austenitizing temperature may prevent this distortion process. In the case of the salt bath or muffle furnace, preheating is not necessarily due to uniform heating of steel.
- Quenching Media
The purpose of hardening treatment is to generate such severe cooling that Pearlitic transformation gets avoided and all austenite converts into a martensitic structure. This severe cooling rate causes excessive internal stresses within the material which results in distortion or quench cracks within the steel. Careful selection of quenching media is important to select the least possible severe cooling rate in order to minimize the generated stresses.
Quenching media selection primarily depends upon the degree of hardenability, size, and shape of the component. Quenchants like Polymer quenchant, salt baths, and hot quenching oils gave much better results in conventional water, oil, or brine solutions.
- Press Quenching
In unconventional steel industries where precision is important and machining allowance is almost negligible, jigs are normally used for press quenching of parts like gears. Within press quenching, platforms are used for preventing shape distortion in components.
- Trays, Fixtures and Support
Over Heating and Burning of Steel
Hot work products of low alloy steel are used widely in the form of fasteners, and machine tools because of properties like high strength, fatigue strength, and good toughness. Improper hot working of low alloy steel imparts a reduction in ductile properties of materials with faceted fracture surfaces making it unsuitable for practical applications. These conditions are normally caused by Overheating or burning of steel.
Details of Over-heating and Burning of steel can be studied in Article, “Over-heating and Burning of steel”.
Temper Embrittlement (Defects of Heat Treatment)
Tempering of steel is carried out to relieve internal stresses in steel and also to bring toughness within a material. Since the process is a sub-critical heating process, so cooling in any manner will not bring any change. Certain compositions of steel if tempered at a temperature of 400oC – 600o
Summary (Defects of Heat Treatment)
|Types of Defects and Characteristics||Causes||Remedies|
|Over Heating||Prolong heating at excessive high temperature||6 times repeated cycles of normalizing|
|Burning||Heating steel for long time near melting point||Not possible to recover|
|Oxidation||Open atmosphere with gases of oxidizing nature||Use of salt bath, muffle furnace, or protective gas environment|
|Decarburization||Open atmosphere with gases of oxidizing nature||Use of salt bath, muffle furnace, or protective gas environment. Use of packing material or cast-iron chips can also be used|
|Excessive hardness of hot-worked annealed steel||Excessive cooling rate||Annealing repeatedly to reduce hardness|
|Black Fracture: steel containing free carbon inclusion||Prolong heating and very slow cooling||Heating steel and forging|
|Dimensional changes after hardening||Martensitic transformation||Slow cooling after Ms or application of surface hardening where possible|
|Warping||Volume change, non-uniform heating, and internal stresses||Use of alloying elements, slow cooling, surface hardening, pre-hardening annealing, and pack quenching|
|Low Hardness After Quenching||Slow cooling, low austenitizing temperature, insufficient soaking time||Annealing or Normalizing and, again, hardening|
|Soft Spots||Vapor blanket stage during quenching, localized decarburization, insufficient cleaning of steel surface||Effective quenchant, pre-quenching annealing, and avoid decarburizing in furnace|
|Excessive hardness after tempering||Insufficient soaking time and low tempering temperature||Second tempering|
|Insufficient hardness after tempering||Tempering temperature is very high||Annealing, re-quenching and re-tempering|
|Quench Cracks||Non-uniform cooling and internal stress|
References (Defects of Heat Treatment)
Heat treatment: Principle and Techniques by C.P Sharma.