Surface Treatment of Steels

Hardening by Carburizing

The primary goal here is to achieve a surface that is hard and wear-resistant while maintaining a tough, ductile core to absorb shock.

The Mechanism This process exploits specific metallurgical factors:

Martensite hardness increases as the carbon percentage increases.
Martensite forms from austenite upon quenching.
Carbon is highly soluble in austenite.
The diffusion/absorption rate increases significantly with temperature.

Process Parameters

Temperature ( $T_{c}$ ): $870 - 95 0^{\circ} C$ .
Time: $10 - 20$ hours.
Applicable Materials: Plain carbon steels ( $< 0.2% C$ ) and low carbon low alloy steels.

Applications Used for components requiring durability under load, such as gears, shafts, bearings, and piston rods.

Outcome

A carbon concentration profile develops along the thickness, achieving maximum Vickers Hardness (HV) on the surface.
The austenite transforms into martensite via oil quenching.

Hardening by Nitriding

Unlike carburizing, this process aims for an extremely hard surface while retaining a tough core.

The Mechanism

Nitrogen forms very hard compounds (nitrides) with Iron.
Nitrogen solubility in ferrite is very low.
At ferrite stability temperatures, diffusion is slow, and the formation of nitrides inhibits further Nitrogen diffusion.

Process Parameters

Temperature ( $T_{N}$ ): $500 - 60 0^{\circ} C$ .
Time: Approx. 70 hours (can range 40-100h).
Chemical Reaction: Ammonia dissociates: $2 N H_{3} \Rightarrow 2 N + 3 H_{2}$ . Atomic Nitrogen then dissolves into the steel.

Applicable Materials

Nitriding steels: Plain medium carbon steel ( $0.4 - 0.6% C$ ) and medium carbon low alloy steels (tempered/semi-finished).
Common alloying elements: Al, Cr, V, Mo.

Structure of the Nitrided Layer

Outer Layer: $(F e, A l, C r)_{2} N$ .
Inner Layer: $(F e, A l, C r)_{4} N$ .
Depth: A thin layer, typically $0.2 - 0.3$ mm.
Hardness: Very high superficial hardness ( $\sim 1100$ HV).

The Fe-N Metastable Phase Diagram

Nitriding Steel Compositions Specific alloys are required for optimal nitriding results.

Steel Type	%C	%Si	%Mn	%Cr	%Mo	%Al	Hardness after Nitriding
30 Cr Mo 10	0.30	0.35	0.60	2.50	0.40	-	650 HV
38 Cr Al Mo 7	0.38	0.30	0.60	1.70	0.25	1.0	1050 HV
42 Cr Al Mo 7	0.42	0.35	0.55	1.70	0.35	0.40	900 HV

Comparison: Nitriding Steels vs. Carburizing

Performance Comparison

Nitrided Steel: Achieves higher surface hardness (up to 1000-1200 HV) but the depth of penetration is shallow ( $< 0.6$ mm).
Carburized Steel: Lower peak surface hardness ( $\sim 800$ HV) but maintains hardness to a greater depth ( $> 1.0$ mm).

Harris’ Formula for Penetration Depth To calculate the depth of the treatment: $Depth (mm) = 660 \cdot t \cdot e^{- \frac{8287}{T}}$ Where $T$ is temperature in Kelvin and $t$ is time in hours.

Post-Treatment Processing

Process Step	Carburized Parts	Nitrided Parts
Pre-treatment	Normalizing + work hardening annealing	Normalizing + Work hardening annealing
Machining	Roughing (medium thickness)	Roughing (medium thickness)
Main Process	Carburizing	Tempering
Hardening	Quenching + tempering at $15 0^{\circ} C$	Finish Nitriding
Finishing	Finish by grinding	Finish by grinding

Induction Hardening

Definition

The process uses alternating current to generate a magnetic field, which creates induced currents in the workpiece. This results in localized thermal energy.

Current Penetration Depth ( $δ$ ) The depth of heating is controlled by the frequency: $δ ≅ \frac{ρ}{π μ f}$

Inductor Types Various shapes exists for different geometries:

Single-shot coils for cylindrical parts.
Shaped coils for slideways or gears.

Slideways vs Coils

Hardness Profiles Induction hardening creates a sharp transition in hardness. For example, in Tempered 4140 steel, hardness holds steady around 700 HV before dropping sharply to the core hardness ( $\sim 330$ HV) at a specific depth determined by the induction frequency and speed.

Heat vs. Chemical Surface Treatments Overview

All surface heat treatments generally confer a compressive stress state on the surface, which increases fatigue resistance.

Process Comparison Table

Process	Steel Type	Component	Conditions
Induction (3 kHz)	0.38% C	Axle	Water Quenched, Tempered at $21 0^{\circ} C$
Flame Hardening	0.67% C	Wheel	Water Quenched, Tempered at $49 0^{\circ} C$
Carburizing	0.2% C (Ni-Cr-Mo alloy)	Gear	Oil Quenched, Tempered at $20 0^{\circ} C$
Laser (15 kW)	0.43% C (Mn alloy)	Gear	Self Quenched
Nitriding	0.2% C (Cr-V-Al alloy)	Gear	Process carried out at $57 0^{\circ} C$

Residual Stress Profiles

Nitriding (Turferrit/Soft Gas): Creates the highest compressive residual stress ( $- 600$ to $- 1000$ MPa) right at the surface.
Induction: Creates deep compressive stress, but lower magnitude at the surface compared to nitriding.
Laser: High surface hardness but shallower stress profile.

Weldability and Heat Affected Zone (HAZ)

When welding steels, the thermal cycle alters the microstructure adjacent to the weld. This is the Heat Affected Zone (HAZ).

Structure of the Welding Zone

Solidified Weld: The melt zone.
Grain Growth Zone: Liquid + $γ$ (austenite) transition.
Recrystallized Zone: $γ$ region.
Partially Transformed Zone: $γ + F e_{3} C$ .
Tempered Zone: Below $A_{1}$ temperature.
Unaffected Base Material: Original microstructure.

Microstructural Evolution The final structure depends heavily on the hardenability of the steel.

At $T_{ma x}$ : The structure becomes Austenite ( $γ$ ) near the melt line.
Cooling (Low Hardenable Steel): Reverts to Pearlite and Ferrite ( $α$ ).
Cooling (High Hardenable Steel): Forms Martensite, which is brittle and prone to cracking.

Weldability Criteria

To be weldable, steel must have low hardenability (low alloying elements).
Steels up to $0.4% C$ are weldable, but caution is needed for higher carbon contents.
Risk: Cracks can form underneath the welding bead in the HAZ.

Summary Tables

Chemical Surface Treatments Summary

Process	Compound	Temp ( $^{\circ} C$ )	Time (h)	Thickness ( $μ m$ )	Hardness (HV)	Distortion
Nitriding	Fe-N	500-550	1-70	50-100	700-1000	Min
Nitro-carb.	Fe-(C,N)	550-650	1-6	50-200	700-1000	Min
Boriding	Fe-B	600-1000	1-4	50-500	1000-1800	High
Carburizing	Fe-C	700-1050	1-46	100-7000	750-950	High
Process QUAD	CrCN, VCN, TiCN	400-700	1-8	5-20	1200-2500	Min

Wear Resistant Ceramic Coatings

Process	Surface Layer	Method	Surface T ( $^{\circ} C$ )	Thickness ( $μ m$ )	Hardness (HV)	Distortion
Cr hard	Cr	Electrolysis	50-80	20-50	700-800	Low
Thermal CVD	TiC, TiCN, TiN	Gas	800-1100	3-15	1500-2000	High
Plasma CVD	TiC, TiN	Arc in vacuum	300-600	1-6	1500-2000	Low
PVD	TiN, CrN	N in vacuum	300-600	1-6	2000-4000	Low
Flame coating	Ni, Cr, B, Si	Powder melting	1000-1100	500-2000	600-800	High
Stellite	Ni, Cr, B, Si	Melting (arc)	$T_{f u s}$	2000-5000	300-900	High

Engineering Notes

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