Heat Treatment of high strength low alloy steels versus tool steels

Heat Treatment of high strength low alloy steels versus tool steels: their design, precautions and application.

Heat treatment is the treatment process that can increase and improve the mechanical properties of the material.

Introductory of high strength low alloy steel and tool steel

High strength low alloy steel

A steel consider as high strength low alloy steel if its manganese content higher than 1.65 percent, or its silicon content is higher than 0.60 percent or its copper content is higher than 0.60 percent. If one of the alloys either manganese, silicon or copper exceeds the content as mentioned above, it is consider as high strength low alloy steel.

High strength low alloy steel has good mechanical properties because of the alloy content in it. High strength low alloy steel has a yield strength more than 275Mpa. It also has lower carbon content which range from 0.05% to 0.25%. Not only that, it has a very good corrosion resistance.

Although high strength alloy steel named as alloy steel but it is not consider as alloy steel. Although it has a bit of alloy content in it, but still it is still categorized as separate steel category, it’s alloy content is to improve the mechanical strength.

There are six types of high strength low alloy steels:

1) Weathering steels – It contains a little bit of alloy contents just to improve its corrosion resistance and
its strength.
2) Microalloyed ferrite-pearlite steels – It has additional of strong carbide added into it for precipitation
strengthening and to make the grain smaller and finer.
3) As rolled pearlitic steels – Carbon manganese steel combines with a little alloys to improve the
mechanical properties and weldability of the steel.
4) Acicular ferrite steels – It has low carbon content which is lesser than 0.05% but it has a very good
strength, mechanical properties and weldability.
5) Dual phase steels – It has a very obvious microstructure of martensite and it has a very good of
ductility and strength.
6) Inclusion shape controlled steels – Ductility and strength are given to the steel by adding some high
toughness elements such as calcium, titanium and so on.

Tool steel

A tool steel has a very high hardness and abrasion resistance. Tool steel has a high carbon content up to 1.2 percent and tool steel contains iron. Manganese content cannot be very high in tool steel if not it will crack while quenching.

There are six types of tool steels:

1) Water hardening ( W grades) – It has a very high carbon content. Since it has high content of carbon,
it will be very hard but very brittle. It might crack when water quench
is carried out.
2) Air hardening ( A grades) – It has a good toughness value and also wear resistance due to high
chromium content.
3) D type ( D grades) – It has high toughness abrasion and wear resistance due to high chromium and
carbon content.
4) Oil hardening ( O grades) – It is quenched by oil and it has a high toughness and abrasion.
5) Shock resist types (S grades) – It has low carbon content but it has high toughness, low abrasion and
will resist shock at different temperature.
6) Hot working ( H grades) – This grade of steel can be cut at high temperature. It has low carbon
content and quite high alloy content to withstand higher temperature.

Heat treatment processes

The basic heat treatment processes include annealing, normalizing, hardening, and tempering.

In a short brief

During heating of metal, the ferrite will transform to austenite. Depends on the heat treatment processes used, if a process with faster cooling is used for example hardening, the austenite will decompose into martensite or pearlite which gives higher strength to the metal.

Heat treatment of alloy steels

Annealing:

For 4037, 4037H alloy steel, the annealing step will be heat to 845 degree Celsius, and cool rapidly to 745 degree Celsius, then at a rate not more than 11 degree Celsius per hour to 630 degree Celsius; or heat to 845 degree Celsius, cool rapidly to 660 degree Celsius and hold for 5 hour to achieve a predominantly pearlitic structure. To get a predominantly spheroidized structure, heat to 760 degree Celsius, and cool from 745 degree Celsius to 630 degree Celsius at a rate not more than 6 degree Celsius per hour; or heat to 760 degree Celsius, cool rapidly to 660 degree Celsius and hold for 8 hours.

For 4140, 4140H alloy steel, the annealing step will be heat to 845 degree Celsius and cool to 755 degree Celsius at a fairly rapid rate, then cool from 755 degree Celsius to 665 degree Celsius at a rate not more than 14 degree Celsius per hour; or heat to 845 degree Celsius, cool rapidly to 675 degree Celsius and hold for 5 hours to achieve a predominantly pearlitic structure. If want to get a predominantly spheroidized structure, heat to 750 degree Celsius and cool to 665 degree Celsius at a rate not more than 6 degree Celsius per hour; or heat to 750 degree Celsius, cool fairly rapidly to 675 degree Celsius and hold for 9 hours.

For 4340, 4340H alloy steel, the annealing step will be heat to 830 degree Celsius, cool rapidly to 705 degree Celsius, then cool to 565 degree Celsius at a rate not more than 8 degree Celsius per hour; or heat to 830 degree Celsius, cool rapidly to 650 degree Celsius and hold for 8 hours to obtain a predominantly pearlitic structure. If want to get a predominantly spheroidized structure, heat to 750 degree Celsius, cool rapidly to 705 degree Celsius, then cool to 565 degree Celsius at a rate not more than 3 degree Celsius per hour; or heat to 750 degree Celsius, cool rapidly to 650 degree Celsius and hold for 12 hours.

For E52100 alloy steel, the annealing step will be heat to 795 degree Celsius and cool rapidly to 750 degree Celsius, then continue cooling to 675 degree Celsius at a rate not exceeding 6 degree Celsius per hour; or heat to 795 degree Celsius, cool rapidly to 690 degree Celsius and hold for 16 hours.

Normalizing:

For 4037, 4037H alloy steel, the normalizing step will be heat the steel to 870 degree Celsius and then cool it in the air.

For 4140, 4140H alloy steel, the normalizing step will be heat to 870 degree Celsius and then cool it in the air same as previous 4037 steel.

For 4340, 4340H alloy steel, the normalizing step will be heat to 870 degree Celsius and then cool it in the air same as previous 4037 and 4140 alloy steel.

For E52100 alloy steel, the normalizing step will be heat the steel to 885 degree Celsius and then cool it in the air.

Hardening:

For 4037, 4037H alloy steel, the hardening step will be heat the steel to 845 degree Celsius and then quench it in oil.

For 4140, 4140H alloy steel, the hardening step will be austenize the steel at 855 degree Celsius and then quench it in oil.

For 4340, 4340H alloy steel, the hardening step will be austenize the steel at 845 degree Celsius and then quench it in oil. Thin section of the steel will be fully hardened by air cooling.

For E52100 alloy steel, the hardening step will be austenize the steel at 845 degree in a neutral salt bath furnace or in a gaseous atmosphere with 1 percent of carbon potential.

Tempering:

For 4037, 4037H alloy steel, the steel is reheated to the temperature required to provide the desired hardness.

For 4140, 4140H alloy steel, the steel is reheated after quenching to obtain the required hardness.

For 4340, 4340H alloy steel, the tempering temperature depends on desired hardness and mechanical properties. Since 4340 steel can be cracked easily during quenching, therefore it must be tempered in the tempering furnace before it reaches ambient temperature.

For E52100 alloy steel, the steel has to be tempered immediately after it has reached the ambient temperature after quenching. 38 to 49 degree Celsius is the best temperature. Due to the high carbon content of E52100 alloy steel, parts must be tempered at least at 120 degree Celsius to convert the tetragonal martensite to cubic martensite.

Heat treatment of tool steels

Annealing:

For the W and S grade tool steels, the annealing temperature must be between 1360F to 1475F. The rate of cooling per hour is between 25 to 40F. The typical annealed hardness must be between 156 to 229F.

For the O and A grade tool steels, the annealing temperature must be between 1400F to 1600F. The rate of cooling per hour is 40F. The typical annealed hardness must be between 183 to 229F.

For the D grade tool steels, the annealing temperature must be between 1600F to 1650F. The rate of cooling per hour is 40F. The typical annealed hardness must be between 217 to 255F.

For the H grade tool steels, the annealing temperature must be between 1550F to 1650F. The rate of cooling per hour is 40F. The typical annealed hardness must be between 192 to 241F.

Normalizing:

Most of the tool steels which contain alloy will not be normalized. Normalize of tool steel may cause cracking on the steel.

Hardening:

For the W and S grade tool steels, rate of heating must be slowly. The preheat temperature must be around 1200 to 1400F. For the hardening temperature, it must be around 1400 to 1700F. The time at temperature should be around 15 to 30 minutes, depends on the steel type. Lastly the quenching medium must be only water or oil.

For the O and A grade tool steels, rate of heating must be slowly. The preheat temperature must be around 1200 to 1450F. For the hardening temperature, it must be around 1450 to 1850F. The time at temperature should be around 15 to 35 minutes, depends on the steel type. Lastly the quenching medium must be only air or oil.

For the D grade tool steels, rate of heating must be very slowly. The preheat temperature must be around 1500F. For the hardening temperature, it must be around 1800 to 1875F. The time at temperature should be around 15 to 45 minutes, depends on the steel type. Lastly the quenching medium must be only air.
For the H grade tool steels, rate of heating must be moderately or rapidly, depends on type of H grade steel. The preheat temperature must be around 1500F. For the hardening temperature, it must be around 1825 to 2300F. The time at temperature should be around 15 to 40 minutes, depends on the steel type. Lastly the quenching medium must be only air, salt or oil.

Tempering:

For the W and S grade tool steel, the tempering temperature must between 350 to 800F depends on the types of W and S grade tool steel. The approximate tempered hardness will be around 40 to 64.

For the O and A grade tool steel, the tempering temperature must between 350 to 1000F depends on the types of O and A grade tool steel. The approximate tempered hardness will be around 57 to 65.

For the D grade tool steel, the tempering temperature must between 400 to 1000F depends on the types of D grade tool steel. The approximate tempered hardness will be around 54 to 61.

For the H grade tool steel, the tempering temperature must between 1000 to 1250F depends on the types of H grade tool steel. The approximate tempered hardness will be around 36 to 55.

Precaution:

During the heating of metal in the furnace, full loads of metal cannot be put into the furnace at the same time. This will cause the non uniform distributed heat among the metals in the furnace. Load of metal at the center of the furnace will not receive much heat compare to the metal located near the wall of the furnace. Therefore, it is hard to control the change of steel during heating or cooling.

Rate of cooling has to be taken care as well, this is because higher cooling rate will lead to more formation of pearlite or martensite. If rate of cooling is not taken care well, it may lead to the production of weak steel.

Rate of heating has to be taken care as well, this is because when the rate of heating is very fast the steel will crack or warp, since sections having different dimensions heat up at different speeds.

During heating, heating of metal beyond its melting point is not allowed, because the metal will undergo pronounced melting. The steel is then said to be burnt.

During salt bathing the steel, a preheating is required to remove any moisture adhere on the steel.

Application of high strength low alloy steels and tool steels:

Application of high strength low alloy steel:

Oil and gas pipelines, heavy duty, heavy duty highway and off shore vehicles, construction and farm machinery, industrial equipment, storage tanks and passenger car components.

Application of tool steel:

Cutting tools, hot extrusion dummy block, die casting dies, battering tools and etc.

Conclusion:

High strength low alloy steels Types of steels tool steels
anneal temperature around 1500F annealing anneal temperature around 1500F to 1550F except W grade tool steel which its anneal temperature is around 1400F slightly lower.
Heated to temperature up to 880 degree Celsius and then let cool in air normalizing Most of the tool steel doesn’t involve normalizing process
Hardening temperature is slightly lower than tool steel hardening Hardening temperature is slightly higher than low alloy steel
tempering temperature needed is low tempering tempering temperature needed is very high

Recommendation:

1) Preheat to about 100 degree Celsius to completely remove moisture before the metal undergo salt bath.

2) Thermocouples should be added into the furnace if the furnace is too big.