14.2 Heat Treatment Processes & Engineering Alloy Steels

13.2 Heat Treatment Processes & Engineering Alloy Steels

Heat treatment encompasses controlled thermal cycles of heating, soaking, and cooling designed to deliberately modify the microstructure, mechanical properties, and residual stress states of engineering alloys without altering component geometry. In heavy machinery, haul trucks, coal washeries, and draglines operated by Coal India Limited, components like gears, drive shafts, crushing rolls, and bucket teeth rely on tailored heat treatment to withstand extreme abrasive wear, shock loading, and cyclic fatigue.

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1. Non-Equilibrium Transformation Kinetics: TTT & CCT Diagrams

While the Iron-Carbon equilibrium diagram dictates phase mixtures under infinitely slow heating and cooling rates, industrial heat treatments involve non-equilibrium kinetic cooling.

Temperature (°C)
  800°C +---------------------------------------------------+
  727°C | A1 Equilibrium Temperature                       |
        |---------------------------------------------------|
  600°C |          \       Pearlite Nose                    |
        |  Austenite\    (Coarse Pearlite)                  |
  500°C |  (Stable)  \                                      |
        |             )  (Fine Pearlite)                    |
  400°C |            /                                      |
        |           /    Upper Bainite (Feathery)           |
  300°C |          /                                        |
        |         /      Lower Bainite (Acicular)           |
  200°C +--------+------------------------------------------+ <--- Ms (Martensite Start)
        |        |       Martensite Transformation          |
  100°C +--------+------------------------------------------+ <--- Mf (Martensite Finish)
        +--------+-------+--------+--------+--------+-------+
       10^-1     1      10       10^2     10^3     10^4    10^5  Time (seconds, log scale)

Time-Temperature-Transformation (TTT) Diagrams

Also designated Bain's curves, S-curves, or Isothermal Transformation (IT) diagrams, TTT charts plot the percentage decomposition of unstable supercooled austenite held at constant subcritical temperatures versus logarithmic time:

1. Pearlite Region ($550^{\circ}\text{C} \to 727^{\circ}\text{C}$): Austenite decomposes via cooperative diffusion into lamellar ferrite and cementite. Near $700^{\circ}\text{C}$, low undercooling produces coarse pearlite (interlamellar spacing $\sim 0.5\text{--}1.0;\mu\text{m}$, softer). Near $550^{\circ}\text{C}$, high undercooling produces fine pearlite (spacing $\sim 0.1;\mu\text{m}$, harder, higher yield strength).
2. Bainite Region ($250^{\circ}\text{C} \to 550^{\circ}\text{C}$): Austenite transforms isothermally into non-lamellar bainite. Upper Bainite ($350\text{--}550^{\circ}\text{C}$) consists of feathery cementite precipitated between ferrite needles. Lower Bainite ($250\text{--}350^{\circ}\text{C}$) features ultra-fine carbide platelets aligned internally at $55^{\circ}\text{--}60^{\circ}$ angles within ferrite plates, displaying superior impact toughness.
3. Martensite Start ($M_s$) & Finish ($M_f$) Temperatures: Below $M_s$ (typically $200\text{--}220^{\circ}\text{C}$ for $0.8%\text{ C}$ steel), the transformation occurs athermally via diffusionless shear. The fraction of martensite formed depends solely on the temperature to which the steel is cooled, independent of holding time:

$M_s (^{\circ}\text{C}) = 539 - 423(\%\text{C}) - 30.4(\%\text{Mn}) - 17.7(\%\text{Ni}) - 12.1(\%\text{Cr}) - 7.5(\%\text{Mo})$

Continuous Cooling Transformation (CCT) Diagrams & Critical Cooling Rate (CCR)

In continuous industrial quenching, the steel cools dynamically across a spectrum of temperatures rather than being held isothermally. Consequently, in a CCT diagram:

• Transformation start and finish curves are shifted to longer times (to the right) and lower temperatures (downward) relative to TTT curves.
• Bainite formation is often completely bypassed in plain carbon steels during continuous cooling.

$\text{Critical Cooling Rate (CCR)} = \frac{T_{\text{austenitizing}} - T_{\text{nose}}}{t_{\text{nose}}}$

The Critical Cooling Rate (CCR) is the minimum cooling velocity required to just suppress the pearlite and bainite transformation "noses", achieving a $100%$ martensitic microstructure upon reaching $M_f$.

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2. Bulk Industrial Heat Treatment Processes

Key Exam Rule for Hypereutectoid Steels: In both Full Annealing and Hardening, hypereutectoid steels are heated only above $A_1$ (not $A_{\text{cm}}$). Heating above $A_{\text{cm}}$ would dissolve all cementite; upon subsequent slow cooling, proeutectoid cementite would re-precipitate as a brittle, continuous network along austenite grain boundaries, destroying ductility.

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3. Tempering & Specialized Isothermal Treatments

Multi-Stage Tempering of As-Quenched Steels

As-quenched martensite contains severe residual macro-stresses and high tetragonality, rendering it excessively brittle for load-bearing service. Tempering involves reheating quenched steel to a subcritical temperature ($150\text{--}650^{\circ}\text{C}$), holding, and cooling to transform BCT martensite into tough tempered martensite (stable BCC ferrite containing fine dispersed spheroidal $\text{Fe}_3\text{C}$ particles).

As-Quenched BCT Martensite (Hard, Brittle)
     │
     ├─ Stage 1 (100–200°C) ──> Loss of tetragonality; ε-carbide (Fe2.4C) precipitates; volume contracts
     ├─ Stage 2 (200–300°C) ──> Unstable retained austenite decomposes into Lower Bainite
     ├─ Stage 3 (300–400°C) ──> ε-carbide transforms to stable cementite (Fe3C) + BCC α-ferrite
     └─ Stage 4 (400–700°C) ──> Spheroidization and coarsening of Fe3C particles (Tempered Martensite)

Temper Embrittlement Phenomena

• Blue Brittleness ($230\text{--}370^{\circ}\text{C}$): Dynamic strain aging due to interstitial carbon/nitrogen interaction with moving dislocations during tempering.
• Reversible Temper Brittleness ($375\text{--}575^{\circ}\text{C}$): Embrittlement observed in Ni-Cr and Mn alloy steels when slowly cooled through $450\text{--}550^{\circ}\text{C}$, caused by segregation of impurity tramp elements (P, Sb, Sn, As) to prior austenite grain boundaries. It is prevented by adding $0.2\text{--}0.5\text{ wt% Molybdenum (Mo)}$ or rapid quenching from $600^{\circ}\text{C}$.

Specialized Isothermal Operations: Austempering vs. Martempering

Temperature (°C)
  Austenitizing Temp +---------------+             +---------------+
                     |               |             |               |
                     |  AUSTEMPERING |             |  MARTEMPERING |
  727°C (A1) --------+---------------+             +---------------+---------
                     |  \            |             |  \            |
                     |   \ Pearlite  |             |   \ Pearlite  |
                     |    \  Nose    |             |    \  Nose    |
                     |     )         |             |     )         |
  Isothermal Hold ───+────(──────────+──           |    (          |
  (250-400°C)        |     \ Bainite |  \          |     \ Bainite |
                     |      \        |   \ Air     |      \        |
  Ms ----------------+-------+-------+----+---     +-------+-------+---------
                     |       |            |        | Equalize Temp | \ Air Cool
  Mf ----------------+-------+------------+---     +---------------+--\------
                     | Microstructure:    |        | Microstructure:   \ (Forms
                     | 100% Tough Bainite |        | Martensite -> Temp) Martensite)

1. Austempering:
   • Steel is austenitized, quenched into a molten salt bath maintained between $250^{\circ}\text{C}$ and $400^{\circ}\text{C}$ (above $M_s$), and held isothermally until complete bainitic transformation is achieved, followed by air cooling.
   • Product: $100%$ Lower Bainite. High impact toughness ($>60\text{ J}$ Charpy), high fatigue limit, zero quench distortion, no subsequent tempering needed.
2. Martempering (Marquenching):
   • Steel is quenched into a hot oil/salt bath just above $M_s$ ($200\text{--}250^{\circ}\text{C}$), held only until temperature equalizes across the entire cross-section (without crossing the bainite start curve), and then slowly cooled through the $M_s\text{--}M_f$ range.
   • Product: Uniform BCT Martensite. Eliminates thermal gradient stresses and prevents quench cracking. Must be followed by conventional tempering.

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4. Thermochemical & Surface Hardening Techniques

Many mechanical components (e.g., mine haul truck transmission gears, pins, and track rollers) require a hard, wear-resistant outer case combined with a ductile, shock-absorbing core.

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5. Metallurgical Roles of Alloying Elements in Steels

Alloying elements shift phase boundaries ($A_1, A_3, A_{\text{cm}}$), retard diffusion kinetics, and alter the mechanical response of engineering steels:

1. Chromium ($\text{Cr}$): Strong carbide former. Shifts the TTT nose to the right, dramatically increasing hardenability. At concentrations $>12\text{ wt%}$, it forms a dense, self-healing passive chromia ($\text{Cr}_2\text{O}_3$) film, producing stainless steels.
2. Nickel ($\text{Ni}$): Powerful Austenite Stabilizer (FCC). Lowers the ductile-to-brittle transition temperature (DBTT), markedly enhancing low-temperature notch toughness and impact resistance (e.g., $9%;\text{Ni}$ cryogenic steels).
3. Molybdenum ($\text{Mo}$) & Tungsten ($\text{W}$): Strong carbide formers ($\text{Mo}_2\text{C}, \text{W}_2\text{C}$). Inhibit grain boundary sliding, elevate creep rupture strength, eliminate temper embrittlement, and confer red hardness (hot hardness up to $600^{\circ}\text{C}$) in High-Speed Steels (18-4-1 HSS: $18%;\text{W}, 4%;\text{Cr}, 1%;\text{V}, 0.7%;\text{C}$).
4. Manganese ($\text{Mn}$): Essential deoxidizer and sulfur scavenger. Combines with sulfur to form benign spheroidal $\text{MnS}$ inclusions, preventing the formation of low-melting-point $\text{FeS}$ grain boundary films that cause hot shortness during forging/rolling. Increases hardenability.
5. Vanadium ($\text{V}$): Forms extremely stable, ultra-fine $\text{VC}$ precipitates that pin austenite grain boundaries during high-temperature austenitizing, ensuring a fine equiaxed grain size (grain refinement) and high yield strength.