liminfoHeat Treatment Reference
Free reference guide: Heat Treatment Reference
About Heat Treatment Reference
The Heat Treatment Reference is a detailed, searchable guide covering all major heat treatment processes used in metallurgical engineering. It includes complete process parameters for annealing (full annealing with furnace cooling at 20-40 C/hr), normalizing (air cooling for grain refinement), quenching (water/oil/polymer quenchant severity H-values), tempering (low/medium/high temperature ranges with resulting hardness), austempering (isothermal bainite transformation at 250-400 C), and induction hardening (frequency vs case depth tables from 1 kHz to 500+ kHz).
The reference provides detailed surface hardening specifications including gas carburizing (900-950 C with carbon potential 0.8-1.0%, case depth vs time following the sqrt(t) law), nitriding (gas nitriding at 500-580 C and plasma nitriding at 350-580 C achieving HV 800-1,200), carbonitriding (simultaneous C+N diffusion at 700-900 C), and flame hardening. Each process entry includes applicable steel grades, resulting hardness values, typical applications, and post-treatment requirements.
Steel-specific heat treatment conditions are provided for commonly used engineering steels: S45C/AISI 1045 (QT to UTS 690-830 MPa), SCM440/AISI 4140 (QT to UTS 930-1,080 MPa), SKD11/AISI D2 (secondary hardening at 500-530 C, HRC 58-60), SKD61/AISI H13 (die casting die at HRC 44-46), SUJ2/AISI 52100 (bearing steel at HRC 62-64), and STS304 solution treatment. The guide also covers TTT/CCT diagram interpretation, Jominy end-quench hardenability testing, and microstructure identification for martensite, pearlite, bainite, ferrite, and austenite.
Key Features
- Complete process parameters for annealing, normalizing, quenching, tempering, and austempering with temperatures, hold times, and cooling methods
- Surface hardening specifications for carburizing (case depth vs time at 930 C), nitriding (HV 800-1,200), carbonitriding, and flame/induction hardening
- Steel-specific heat treatment tables for S45C, SCM440, SKD11, SKD61, SUJ2, and STS304 with tempering hardness curves
- Quenchant severity comparison: water (H=1.0-5.0), oil (H=0.25-0.70), polymer (H=0.20-1.00), and air (H=0.02)
- TTT and CCT diagram interpretation with critical cooling rates and microstructure prediction by cooling method
- Jominy end-quench test guide with J-distance vs cooling rate correlation and hardenability comparison across steel grades
- Microstructure identification covering martensite (BCT, HRC 60-67), pearlite, bainite (upper vs lower), ferrite, and austenite
- Practical application mapping: gear teeth (induction hardening), bearings (SUJ2 QT + sub-zero), dies (SKD11/SKD61 multi-temper cycles)
Frequently Asked Questions
What is the difference between annealing and normalizing?
Both involve heating above Ac3 + 30-50 C, but annealing uses slow furnace cooling (20-40 C/hr) producing coarse pearlite (softer, better machinability, HB 150-200), while normalizing uses air cooling producing fine pearlite (higher strength and hardness, HB 200+). Normalizing is used to refine grain structure after casting or forging, while annealing is used when maximum softness is needed for machining.
Why is tempering required after quenching and what temperature should I use?
Quenching produces martensite which is extremely hard (HRC 60-67) but brittle and prone to cracking. Tempering restores toughness: 150-200 C for cutting tools and bearings (retains HRC 60-64), 350-500 C for springs (HRC 40-50, maximum elastic limit), and 500-650 C for gears and shafts (HRC 25-35, optimal toughness). Avoid the temper embrittlement zone at 300-350 C for alloy steels.
How do I choose between carburizing and nitriding for surface hardening?
Carburizing (900-950 C) produces deeper case depths (0.8-1.5 mm) with HRC 58-63, suitable for gears and bearings using low-carbon steels (SCr420, SCM420). Nitriding (500-580 C) produces shallower cases (0.2-0.7 mm) but higher hardness (HV 800-1,200) with minimal distortion since it is performed below the transformation temperature. Choose nitriding when dimensional precision is critical, such as injection molds and precision shafts.
What are the recommended heat treatment conditions for SCM440 (AISI 4140)?
For QT treatment: austenitize at 850 C, oil quench (critical diameter ~50 mm in oil), then temper at 550-650 C for 1-2 hours. This produces UTS 930-1,080 MPa with YS above 830 MPa. Tempering hardness: 200 C gives HRC 50-53, 400 C gives HRC 42-46, 600 C gives HRC 28-33. Avoid tempering at 300 C due to temper embrittlement in Cr-Mo steels.
How do I read a CCT diagram to predict microstructure?
Plot the actual cooling curve on the CCT diagram. Where the curve crosses transformation regions determines the resulting microstructure. For S45C: furnace cooling (~20 C/hr) gives coarse pearlite (HB 150), air cooling (~100 C/min) gives fine pearlite (HB 200), oil quenching (~300 C/min) gives martensite + bainite (HRC 45), and water quenching (~600 C/min) gives full martensite (HRC 55). The critical cooling rate is the minimum rate that avoids the nose of the C-curve.
What is the Jominy test and how do I use the results?
The Jominy end-quench test measures hardenability by water-quenching one end of a 25mm x 100mm specimen and measuring hardness along its length. Results are expressed as J-distance = hardness (e.g., J5 = HRC 52 means HRC 52 at 5mm from the quenched end). J-distances correlate to cooling rates: J1.5 ~ 300 C/s (water surface), J10 ~ 20 C/s (oil center), J40 ~ 2.5 C/s (air cool). Compare hardenability: S45C drops rapidly (J10=HRC 28) while SCM440 maintains hardness (J10=HRC 50).
What is the difference between upper and lower bainite?
Upper bainite forms at 350-550 C with a feathery/plate structure, HRC 40-45, and poor toughness (should generally be avoided). Lower bainite forms at 250-350 C with an acicular structure, HRC 45-55, and excellent toughness comparable to tempered martensite. Austempering specifically targets lower bainite by isothermal transformation in a salt bath at 250-400 C, producing parts with minimal distortion and no separate tempering step needed.
How do I heat treat SKD11 (AISI D2) tool steel for die applications?
Use two-stage preheat (650 C + 850 C), austenitize at 1,030 C, then air cool or gas quench to HRC 62-64. For die applications, temper at 520 C twice (minimum 2 cycles required) to achieve HRC 58-60 via secondary hardening. For cutting tools, temper at 180 C for HRC 61-63. Sub-zero treatment at -80 C transforms retained austenite (typically 8-12%) for improved dimensional stability. Always use multi-stage preheating to prevent thermal shock cracking.