Steel is considered one of the most common heat-treated materials due to its wide range of use. Heat treatment and especially quenching is always accompanied by a large rejection rate. In the present article, we are sharing common quenching and tempering defects, their causes, and possible remedies. Here we are only going to discuss the Overheating of steel and the Burning of steel. For a detailed article on Defects of Heat treatment, Follow “Various Defects of Heat treatment and their remedies”.
Read the below articles to study important aspects of the TTT diagram, Phase diagram in steel, and alloying elements effects in steel.
- Microstructure Scaling Methods – Scaling Microstructure
- Steel Quenching Methods – Quenching Steel
- The austenitizing temperature in Steel – Importance of Austenite in Steel
- TTT diagram of steel – Phases of TTT diagram
- Effect of Alloying elements in Steel – TTT diagram and Phase Diagram
- Steel Tempering Process – Tempering stages, and Temper colors
- Various Defects of Heat treatment in steel – Defects and Remedies in steel
Common Improper heat treatment practice
In the quenching process, faster cooling rates and the presence of alloying elements cause lots of hurdles for metallurgical engineers to prevent cracking or distortion in steels. Major hurdles faced by engineers in achieving a good quality heat-treated steel are;
- Thermal and structural stresses
- Metallurgical defects
- Improper material selection
- Wrong materials
- Defects developed during cold or hot working of materials
- Tempering defects
- Poor structural design of steel
Some problems like poor structural design can be easily distinguished with the naked eye, but few defects will generate no visual change in heat-treated steel that’s metallurgical investigations are needed after every heat treatment of steel for quality inspection.
Burning of Steel and Over heating 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 Over heating of steel or Burning of steel.
Over Heating of Steel
Low alloy steel containing microalloying elements like Manganese, Sulphur, phosphorus, and silicon are more prone to over heating of steel or burning of steel.
In the case of Low-alloy steel, loss in mechanical properties along with matte facets and dimple type ductile fractures in the result of impact loading give rise to phenomena of Over-heating of steel.
During the Pre-heating of Steel to 1200oC, MnS inclusions precipitate around austenite grains. After the cooling stage, this network of MnS precipitates provides a weaker low-energy inter-granular path to follow rather than a transgranular path. After impact loading,
Over heating of steel may be present during hot working, casting and welding zones of steel.
Burning of Steel
If pre-heating temperature prior to hot working conditions raises to 1400oC, Over-heating becomes severe and converts into Steel Burn. Forge products after steel burn often break upon cooling from forging or during heat treatment. Burning of steel is considered a severe form of Over heating of steel.
Heating above 1400oC, localized melting occurs due to the segregation of low-melting microalloying elements like phosphorus, Sulphur, and Manganese at grain boundaries of austenite grains. During the cooling stage, MnS dendrites will form in phosphorus-rich grain boundary region which makes structure extremely weak making steel prone to cooling quench cracks.
Detection of Over heating of steel and Burning of steel
Two most common methods normally used for detection are;
- Fracture Testing
Fracture test is a common method employed for the detection of the burning of steel and over heating of steel. It is mentioned before, facets appear in dimple type fractures around the grain boundary area. These facets in fracture surfaces are a result of ductile dimples which are nucleated by fine MnS array in the grain boundary area. Fracture testing is commonly used for over-heated steels. Burning and over heating of steel can be differentiated by the metallography technique.
- Metallography
Metallography is one of the swiftest methods in the detection of over-heated steel. Etchants react with matrix or grain boundary area to create a contrast along with etching pits shown above in the picture.
In the below table, common etchants are shared along with their application procedure. In order to differentiate between over-heated and burned steel, etchants used are also shared in the below table. These etchants will generate an opposite contrast for over-heated and burned steel.
| Etchant | Application procedure | Action on Over-heating steel | Action on Burned steel |
|---|---|---|---|
| 2.5% Nital | Place sample for 30s and wash after it. | Gives only grain contrast, do not generate pits, that’s why not a good reagent for over-heating indication | White boundaries giving idea of pre-existing grains |
| Saturated ammonium nitrate aqueous solution | Specimen Anode with current density 1.0 A cm-2 | White boundaries giving idea of pre-existing grains | Black boundaries outlining austenitic grains |
| 10% Sulphuric acid + 10% Nictric acid in water | Insert sample for 30s and wash the surface. Repeat three times and then repolish lightly | Black boundaries outlining austenitic grains | White boundaries giving idea of pre-existing grains |
Factors affecting Burning of steel and Over Heating of steel
Factors which affect burning of steel and over heating of steel are;
- Composition of low alloy steel
- Cooling rate
- Methods of Manufacture
Composition of low alloy steel: Low alloy steel contains sulfur and phosphorus inclusions which are the main reason for over heating of steel and burning of steel. With an increase in sulfur content, precipitation of sulfide content increases thereby decreasing mechanical properties. Minimal content for precipitation of sulfide requires 0.02% Sulpher.
For the burning of steel, phosphorus is considered a major contributor. With the same phosphorus content and higher sulfur content, the overheating temperature goes up while burning temperature goes down. We explained previously, overheating temperature for mild steel is 1200o C while the burning temperature is 1400o C. With higher phosphorus content this burning temperature comes down and causes quench cracks. Burn steel microstructure is not recoverable while controlled heat treatment which will be discussed in remedy can be used for over heated steel.
Temperature: Austenitizing temperature is of prime importance for hot working. Too high working temperature causes over-heating.
Cooling rate: Intermedia cooling rate in between 10o C and 100oC/min can become harmful in the over-heating temperature range. As, in this range, the precipitate can get evenly distributed as a fine layer around austenite grains.
The very slow cooling rate can cause clustering of MnS precipitates thereby reducing effect on impact strength.
The very high cooling rate will prevent the growth of nucleated MnS precipitates thereby making them less damageable to steel mechanical properties.
Manufacturing method: The main culprit in over heating of steel and burning of steel is impurities like Sulfur and Phosphorus. These impurities can be minimized with electro-slag melting. The electro-slag manufacturing process produces steel with minimal sulfur content thereby reducing the effect of MnS precipitates.
Prevention of Over-heating of steel and Burning of Steel
- Initially, choose hot-working temperature below over-heating temperature for specific steel in order to prevent steel from burning.
- The addition of alloying elements like Calcium, Cerium, and Zirconium can increase over-heating temperature and, also, the mechanical properties of steel. Higher Ce/S ratio prevent elongated MnS precipitates and convert into globular precipitates which prevents inter-granular weak fracture. Burning can be avoided by the addition of Zirconium and calcium. They form less-soluble sulfides which are refractory sulfide in nature. Since they don’t get dissolved in the melted region, that’s why they prevent the burning of steel.
Over-heated steel Reclamation
We previously, discussed, burn steel morphology is difficult to recover due to sulfides dissolved in segregated low-melted alloying elements in the grain boundary area. Whereas sulfides nucleated in an over-heating temperature range in the form of elongated structure can be restored by the following mechanism;
- With Repeated Normalizing; Perform normalizing cycles minimum 6 with each cycle having austenitizing temperature 10oC greater than the previous one.
- Oil hardening and tempering repeatedly after austenitizing in a carburizing atmosphere at a temperature of 900-1000oC. After hardening, check the microstructure. After 3 repeated hardening stages, if steel still not get recovered than recovery is difficult.
- Soaking steel at over-heating temperature allowing clustering of MnS precipitates thereby reducing their impact on mechanical properties. Too much soaking can cause dimensional changes and the purpose of forging to achieve final dimensional accuracy gets diminished.
References
Heat treatment: Priniciple and Techniques by C.P Sharma.
