Quenching And Tempering Of Steel Pipes

Heat the steel to a red-hot state (with its internal structure transforming into "austenite"), then rapidly cool it (into water/oil/other medium), preventing the internal structure from "slowly reverting" and locking it into a hard and brittle "martensite".

1. Heating (austenitization) - Heat the steel to around 800-950°C, causing the internal atoms to rearrange into the high-temperature structure "austenite". This is the prerequisite; insufficient temperature means it's a waste of time.
2. Holding - Allow the heat to penetrate the core to ensure the entire piece is thoroughly heated and the structure is uniform.
3. Rapid cooling - This is the essence of the "quenching" word. The key is speed: cool it down within seconds to tens of seconds. Not enough time for carbon atoms to re-distribute and arrange into soft tissue (lamellar pearlite/ferrite), so the atoms are "frozen" in an oversaturated, distorted lattice, which is martensite - it is extremely hard but also extremely brittle.

For example: Just like boiling a pot of water (austenitization), if it is allowed to cool naturally, it will form regular ice (soft tissue); if it is immediately frozen by pouring it into liquid nitrogen, the water molecules cannot queue up and form disordered hard ice (martensite). Quenching is like "instant freezing".
Key properties:

• After quenching, the steel becomes harder, stronger, and more wear-resistant (thanks to martensite).
• But it also becomes brittle and has high internal stress, making it prone to cracking when used directly.
• Therefore, quenching is almost never used alone; it must be immediately followed by a tempering process (reheating at a low temperature and holding for a while), to reduce brittleness and internal stress and restore toughness. Quenching + tempering = tempering treatment, which is the actual thing used in engineering.

So remember a mnemonic:

Quenching is "hardening", tempering is "rescuing brittleness", they are a pair, both indispensable.

The process of tempering is much "gentler" than quenching - there is no need for rapid cooling. The key parameters are three: tempering temperature, holding time, and cooling method after出炉. Among them, temperature determines everything.

For steel pipes, the common tempered pipes you often see are those that undergo high-temperature tempering： after quenching, temper at 550~650℃ to obtain "high strength + good toughness" sorbite structure.

1. Temper immediately after quenching - the principle is "quench immediately and temper", it is best to transfer to the tempering furnace as soon as the workpiece is not completely cooled to avoid cracking due to prolonged quenching state.

2. Load into the furnace for heating - the workpiece is loaded into the furnace and heated or placed in the furnace at the target temperature. The temperature is selected according to the target structure in the table, such as 550~620℃ for high-temperature tempering.

3. Holding (key parameter) - allow the transformation of the structure to be sufficient and the internal stress to be uniformly eliminated.

The empirical formula is: 

For carbon steel and alloy steel, generally estimate by 1.5~2.5 minutes/mm of effective thickness;
Also commonly use "≥2 hours as the lower limit" for insulation;
Alloy steel requires more thorough insulation than carbon steel because alloy elements diffuse slowly.

4. Unloading and cooling - the cooling method after tempering has a relatively small impact. The common practice is:
Carbon steel: air cooling is sufficient;
Alloy steel (especially those containing Cr, Mn, Ni) need to be slowly cooled (in furnace or sand cooling) to prevent the second type of tempering brittleness.

5. Inspection - use a hardness tester to inspect whether the hardness is within the target range to determine if the tempering temperature is appropriate (if the hardness is too high, increase the tempering temperature next time; if it is too low, reduce it).

Pit Explanation
Avoid the tempering brittleness zone Part of alloy steel will experience a sudden drop in impact toughness at 250-400℃ (first type) and slow cooling (second type) - select a gentle cooling or avoid this zone
Insufficient holding time Core does not fully temper, outer is soft and inner is hard, uneven performance
Temperature deviation 20-30℃ difference in high-temperature tempering can cause the hardness to drop several HRC, uniformity of furnace temperature is very important

Heating (Austenitization 800~950℃) → Holding → Rapid cooling (water/oil/PAG) → [Quenching is completed, obtained martensite]
→ Immediately transfer to the tempering furnace → Hold at the target temperature (such as 580℃×2h) → Air cooling/Slow cooling → [Quenching and tempering are completed, obtained sorbite]
→ Hardness/Performance inspection
A mnemonic： Quenching determines "hardness", tempering determines "toughness", temperature selection is accurate, time is sufficient, cooling depends on the steel type.