Re: What changes take place to 0.3% carbon steel when heated to 1400degrees C

There are several good references for the effects of heat treatment on steels. A few are listed below.

At 1400°C, 1030 carbon steel (Fe - 0.27-0.34 wt.% C - 0.60-0.90 wt.% Mn) will be in the austentite phase region according to the Fe-C phase diagram. Assuming no change in chemistry due to the welding process, the microstructure evolved would depend on the cooling rate. Time-temperature-transformation (T-T-T) diagrams will give the specifics.

If the cooling rate is slow, then the phase diagram would be applicable. The material would begin transforming from austenite to ferrite around 825°C with complete transformation occuring at 738°C. Below 738°C the alloy would have a pearlitic structure consisting of austenite and cementite (Fe₃C)lamallae. The slower the cooling rate, the coarser the microstructure.

At intermediate cooling rates, bainite forms. The bainite also has austenite and cementite, but the cementite tends to be either laths (upper bainite) or discrete particles (lower bainite). The biggest differences between bainite and pearlite are the mechanism by which the cementite forms and a change from a lamellar structure to discrete cementite particles.

At the fastest cooling rates, martensite will form. This is a metastable compound that is extremely hard and brittle. It is the base structure for hardened steels. It has a microstructure that can be clasified as lath, plate or a mixture of the two. By subsequent tempering the toughness can be increased at the expense of hardness.

Further heat treatment such as raising the temperature back into the austenitic range (>825°C) will allow for better control of the final weld microstructure. However, this is often impractical.

Other properties are also affected by the heat treatment. The grain size tends to increase with slower cooling rates. There is a change in magnetic properties around 770°C, the Curie temeprature for pure Fe. At the higher temepratures the Fe is non-magnetic while below the Curie temperature it is magnetic.

References

• Metals Handbook - Desk Edition, ASM International, Materials Park, OH (1985) pp.28.2 - 28.10
• Iron Carbon Equilibrium Phase Diagram (at 0.3% carbon content)
• Steels - MMM211 Materials Science II lecture
• Phase Diagram Tutorial - University of Arizona MSE Department
• British Steel - Microstructure examples
• The Making, Shaping and Treating of Steel, United States Steel Corp., Pittsburgh, PA (1957) pp.788-815 - includes T-T-T curve for 1035 steel (p. 802)