Heat Treatment and Surface Engineering

Introduction and Context

Heat treatment and surface engineering are central manufacturing strategies for controlling the performance of metallic components. Heat treatment modifies the internal structure of a material by controlled heating, holding, and cooling. Surface engineering modifies only the surface or near-surface region, where wear, friction, corrosion, contact fatigue, and oxidation usually originate.

The central engineering principle is that component performance depends on both bulk properties and surface properties. A gear, for example, may require a tough core to resist shock and a hard surface to resist wear. A bearing may require high hardness, dimensional stability, low friction, and resistance to rolling-contact fatigue. These requirements are rarely achieved by composition alone; they are obtained through a carefully selected thermal cycle and surface treatment.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain phase transformations in steels and their relationship to thermal cycles.
2. Distinguish annealing, normalizing, hardening, and tempering.
3. Select suitable case-hardening methods for engineering applications.
4. Compare coatings, plating, thermal spraying, and surface-modification processes.
5. Relate surface treatments to wear, corrosion, fatigue, dimensional accuracy, and cost.
6. Identify defects, process limitations, safety requirements, and quality-control methods.

1. Metallurgical Basis of Heat Treatment

1.1 Phases in iron–carbon alloys

Pure iron changes crystal structure with temperature. At room temperature it is primarily body-centred cubic ferrite, denoted α-iron. Above approximately 912 °C, it becomes face-centred cubic austenite, denoted γ-iron. At still higher temperature it becomes body-centred cubic δ-ferrite before melting.

Carbon has much greater solubility in austenite than in ferrite. This difference in solubility is the basis of steel heat treatment. In plain-carbon steels, austenite may transform into several products depending on composition, cooling rate, and transformation temperature:

• Pearlite: alternating ferrite and cementite; moderate strength and ductility.
• Bainite: fine ferrite–carbide structure; generally stronger and tougher than pearlite.
• Martensite: supersaturated, distorted body-centred tetragonal structure; very hard but relatively brittle before tempering.
• Spheroidized cementite: rounded carbide particles in ferrite; excellent machinability and ductility.

The eutectoid composition is approximately 0.76 wt% carbon at about 727 °C. A eutectoid steel transforms from austenite into pearlite during sufficiently slow cooling. Hypoeutectoid steels contain less carbon and form proeutectoid ferrite before pearlite; hypereutectoid steels form proeutectoid cementite before pearlite.

1.2 Thermal cycles and transformation diagrams

A heat-treatment cycle consists of:

1. Heating at a controlled rate.
2. Holding at a selected temperature to obtain the required phase or chemical reaction.
3. Cooling at a controlled rate and in a selected medium.

The choice of cooling rate is critical. Time–temperature–transformation (TTT) diagrams describe transformation at approximately constant temperature, whereas continuous-cooling-transformation (CCT) diagrams describe transformation during continuous cooling, as occurs in industrial furnaces and quenching systems.

The critical cooling rate is the minimum rate required to avoid diffusional products such as pearlite and obtain predominantly martensite. It depends on composition, grain size, austenitizing temperature, and prior microstructure. Alloying elements such as chromium, molybdenum, manganese, and nickel generally increase hardenability by shifting transformation curves to longer times. Hardenability is not the same as hardness: hardness is a local property, while hardenability describes the depth to which a steel can harden.

1.3 Heat-transfer considerations

During quenching, the surface cools before the centre. This produces temperature gradients and transformation stresses. If the difference between the thermal and transformation strains becomes excessive, distortion or cracking may occur. Quenching severity depends on the medium, agitation, workpiece geometry, and temperature.

Common quenchants include water, brine, oil, polymer solutions, and gas. Water and brine provide high cooling rates but greater cracking risk. Oil is slower and usually produces less distortion. Polymer solutions allow adjustable cooling characteristics. Gas quenching, especially under high pressure, provides clean processing and good dimensional control but requires suitable alloy steels.

2. Principal Heat-Treatment Processes

2.1 Annealing

Annealing generally involves heating to a suitable temperature, holding, and slow cooling, often inside the furnace. Its purposes include reducing hardness, improving machinability, relieving residual stress, refining structure, and increasing ductility.

Types include:

• Full annealing: produces relatively coarse pearlite and ferrite in carbon steels and is used to soften material.
• Process annealing: applied mainly to low-carbon steels to restore ductility after cold working.
• Stress-relief annealing: conducted below the transformation range to reduce residual stresses without substantially changing phase composition.
• Spheroidizing anneal: produces rounded cementite particles and is especially useful before machining or hardening high-carbon steels.

Annealing improves dimensional stability but is relatively slow and energy-intensive.

2.2 Normalizing

Normalizing consists of heating steel above its critical transformation temperature followed by cooling in still air. It generally produces finer pearlite and ferrite than full annealing. Compared with annealed steel, normalized steel usually has higher strength and hardness, improved structural uniformity, and better resistance to some forms of fatigue. It is widely used for forgings, castings, weldments, and medium-carbon steel components.

2.3 Hardening

Hardening involves austenitizing followed by rapid cooling sufficiently fast to form martensite. The austenitizing temperature must be high enough to dissolve the desired amount of carbon and alloy carbides, but excessive temperature can cause grain growth, oxidation, decarburization, and distortion.

The maximum achievable hardness is primarily governed by carbon content. Hardening depth is governed by hardenability, section size, and quenching conditions. A thick component may have a hard martensitic surface but a softer core if the cooling rate at the centre is inadequate.

2.4 Tempering

As-quenched martensite is hard but contains high residual stress and has limited toughness. Tempering reheats the hardened steel to a temperature below the eutectoid temperature, holds it, and cools it. Carbon diffuses from supersaturated martensite, producing tempered martensite and relieving stress.

Low-temperature tempering retains high hardness for tools and wear parts. Medium-temperature tempering provides a useful balance of strength and toughness for springs and shafts. High-temperature tempering produces tempered sorbite and is used where toughness and fatigue resistance are important, such as in structural machine components.

A simplified tempering parameter is sometimes used for comparing time–temperature combinations:

Here, P is the tempering parameter, T is absolute temperature in kelvin, t is time in hours, and C is an empirical material constant, often near 20 for steels. This relationship is empirical, not universal; it should be used only within the temperature and time range for which the material constant was established.

2.5 Austempering and martempering

Austempering quenches austenitized steel to an intermediate temperature and holds it until bainite forms. It can provide good toughness and reduced distortion compared with direct quenching, but it requires adequate hardenability and section size.

Martempering interrupts quenching near the martensite-start temperature, equalizes temperature throughout the part, and then cools through the martensitic range. It reduces thermal gradients and distortion but does not eliminate transformation stresses.

3. Case Hardening and Surface Hardening

Case hardening creates a hard surface layer over a tougher, more ductile core. It is appropriate for gears, cams, pins, bushes, and shafts subjected to contact stress and impact.

3.1 Carburizing

Carburizing introduces carbon into a low-carbon steel surface at elevated temperature, usually in a gas, vacuum, or controlled atmosphere. After carburizing, the part is quenched and often tempered. The surface becomes high-carbon martensite while the core remains comparatively tough.

Carburizing is suitable for deep cases and large production volumes. Its disadvantages include high temperature, distortion, long cycle time, and the need for accurate atmosphere control. Vacuum carburizing reduces oxidation and can provide improved uniformity.

3.2 Carbonitriding

Carbonitriding introduces both carbon and nitrogen, generally at a lower temperature and shallower depth than carburizing. It is commonly applied to small components requiring improved wear and fatigue resistance. The process can produce a hard case with reduced distortion, but excessive retained austenite or compound-layer formation must be controlled.

3.3 Nitriding

Nitriding introduces nitrogen into alloy steel at approximately 500–580 °C, usually without quenching. Alloying elements such as aluminium, chromium, molybdenum, and vanadium form hard nitrides. Nitriding produces a hard, wear-resistant case with excellent dimensional stability and compressive residual stress.

Its limitations are relatively shallow case depth, long treatment time, and the requirement for a suitable alloy composition. A brittle compound layer, sometimes called a white layer, may be harmful if it is too thick.

3.4 Nitrocarburizing

Nitrocarburizing introduces nitrogen and a smaller amount of carbon, often in salt baths, gas systems, or plasma systems. It produces a thin compound layer and a diffusion zone. The process is effective for improving sliding wear, scuffing resistance, and corrosion resistance of steels and cast irons.

3.5 Induction and flame hardening

Induction hardening uses electromagnetic induction to heat the surface rapidly above the austenitizing temperature, followed by quenching. Flame hardening uses a fuel-gas flame for the same purpose. Both retain a tough core and can produce selective hardened patterns.

Induction hardening is highly controllable and suitable for automated production, but coil design and power frequency must match the required heating depth. Flame hardening is flexible for large parts but requires careful operator control and presents greater risk of nonuniform heating.

4. Surface Engineering Technologies

Surface engineering includes deposition, conversion, diffusion, and mechanical modification. The correct process depends on the dominant failure mechanism, substrate, operating temperature, geometry, allowable dimensional change, and environmental regulations.

4.1 Electroplating and electroless plating

Electroplating deposits metal by passing direct current through an electrolyte. Chromium, nickel, zinc, copper, and precious metals are common deposits. Zinc plating protects steel primarily by sacrificial corrosion protection, whereas hard chromium can improve wear resistance and dimensional restoration.

Electroless plating uses a chemical reducing agent rather than an external electrical current. Electroless nickel can provide relatively uniform coverage on complex geometries and may contain phosphorus or boron to modify hardness and corrosion resistance.

Important controls include surface preparation, bath chemistry, current density, temperature, hydrogen absorption, adhesion, thickness uniformity, and post-treatment. Hexavalent chromium processes are effective but raise serious occupational and environmental concerns; trivalent chromium and other alternatives are increasingly adopted.

4.2 Conversion coatings

Conversion coatings chemically transform the substrate surface. Examples include anodizing of aluminium, phosphating of steel, black oxide, and chromate conversion coatings. They can improve corrosion resistance, paint adhesion, lubricity, or appearance. Anodizing produces a hard aluminium-oxide layer, although the oxide can be brittle and porous unless sealed.

4.3 Thermal spraying

Thermal spraying propels molten or semi-molten particles onto a prepared surface. Major processes include flame spraying, electric-arc spraying, plasma spraying, high-velocity oxy-fuel spraying, and cold spraying.

The deposited coating is formed from flattened particles called splats. Coating properties depend on particle temperature and velocity, substrate preparation, porosity, oxide content, residual stress, and post-treatment. Thermal spraying can restore worn dimensions and deposit ceramics, metals, cermets, and polymers. Because bonding may be primarily mechanical, grit blasting and controlled surface roughness are essential.

4.4 Physical and chemical vapour deposition

Physical vapour deposition (PVD) deposits materials such as titanium nitride, chromium nitride, and diamond-like carbon in a vacuum using physical vaporization and ion assistance. Chemical vapour deposition (CVD) forms coatings through chemical reactions in a gas phase, often at higher temperatures.

PVD coatings are thin, hard, and useful for cutting tools and forming dies. CVD can provide excellent conformity and wear resistance but may impose higher substrate temperatures. Coating adhesion, edge preparation, substrate hardness, and coating residual stress strongly influence performance.

4.5 Diffusion coatings

Processes such as aluminizing, chromizing, and boriding diffuse an element into the substrate. Boriding can produce extremely hard boride layers, while aluminizing improves oxidation resistance at elevated temperature. Diffusion treatments provide strong metallurgical bonding but may produce brittle phases or excessive dimensional change.

4.6 Mechanical surface modification

Shot peening bombards a surface with small particles, producing plastic deformation and compressive residual stress. This can improve bending and contact fatigue resistance by delaying crack initiation and growth. Excessive intensity, poor coverage, surface contamination, or sharp notches can negate the benefit.

Laser surface hardening, laser cladding, and laser shock peening provide localized treatment with accurate energy control. These technologies are attractive for repair and high-value components, although equipment cost and process qualification can be substantial.

5. Matching Treatment to Failure Mode

Surface hardness alone is not a sufficient selection criterion. A very hard coating may fail by brittle cracking or delamination if the substrate is soft, the coating is poorly bonded, or the contact stress is high. Tribological design must consider the coating, substrate, lubricant, counterface, load, speed, temperature, and contaminants as a system.

6. Quality Control and Defects

Common heat-treatment defects include quench cracks, soft spots, excessive retained austenite, decarburization, oxidation, grain coarsening, distortion, and dimensional growth. Surface-treatment defects include poor adhesion, porosity, uneven thickness, pinholes, blistering, hydrogen embrittlement, residual tensile stress, and coating delamination.

A robust quality plan may include:

• hardness and microhardness profiles;
• case-depth measurement by metallography or hardness transition;
• chemical-composition and atmosphere monitoring;
• dimensional inspection before and after treatment;
• magnetic-particle, dye-penetrant, ultrasonic, or eddy-current inspection;
• coating-thickness and adhesion tests;
• roughness, residual-stress, and porosity measurements;
• representative fatigue, wear, or corrosion testing.

The effective case depth must be defined explicitly, because different standards use different hardness thresholds and measurement procedures. Process records should identify material heat, furnace cycle, quench conditions, bath chemistry, equipment calibration, and inspection results.

7. Applied Case: Carburized Transmission Gear

Background

An automotive transmission gear made from a low-carbon alloy steel must resist repeated tooth contact, bending fatigue, and occasional shock loading. A uniformly hardened high-carbon steel would provide hardness but insufficient toughness and would be difficult to manufacture without distortion.

Problem

The gear requires a hard tooth surface, a tough core, controlled dimensional change, and reliable fatigue performance after grinding.

Analysis

The dominant surface failure risks are pitting, scuffing, and adhesive wear. The dominant bulk failure risk is tooth-root bending fracture. Therefore, a hard case alone is not enough; the core must retain strength and toughness. The component geometry also makes quenching distortion important.

Approach

The selected sequence is:

1. Machine the gear from low-carbon alloy steel.
2. Carburize in a controlled atmosphere or vacuum system to the specified effective case depth.
3. Quench using a controlled oil or gas-quenching cycle.
4. Temper at a low temperature to reduce residual stress while retaining high surface hardness.
5. Shot-peen selected tooth-root regions if fatigue testing justifies it.
6. Finish-grind the teeth to remove distortion and achieve the required profile.
7. Verify surface hardness, core hardness, case depth, dimensional accuracy, microstructure, and crack freedom.

Outcome

The hard martensitic case improves resistance to contact fatigue and wear, while the lower-carbon core absorbs shock and supports the tooth. Controlled quenching and finish grinding manage distortion. If retained austenite is excessive, dimensional instability may occur during service; therefore, the process may require sub-zero treatment, adjusted carbon potential, or a modified tempering schedule.

Learning points

The case demonstrates that treatment selection begins with failure analysis rather than with a preference for a particular process. It also shows that heat treatment, finishing, inspection, and design geometry must be considered as one manufacturing system.

8. Environmental, Safety, and Professional Considerations

Heat-treatment equipment presents burn, fire, furnace-atmosphere, and quench-vapour hazards. Operators must control combustible atmospheres, provide ventilation, use appropriate personal protective equipment, and follow lockout and emergency procedures. Salt baths and plating operations can expose workers to corrosive or toxic chemicals.

Environmental decisions include energy consumption, water use, quenchant disposal, volatile emissions, heavy-metal waste, and substitution of hazardous chemicals. Engineers should apply life-cycle thinking: a more energy-intensive treatment may be justified if it substantially extends component life, but claims must be supported by data.

Hydrogen embrittlement is a serious risk for high-strength steels exposed to acid cleaning or electroplating. Appropriate cleaning, process control, baking where specified, material selection, and inspection are necessary. Compliance with applicable occupational, environmental, and product standards is part of professional engineering responsibility.

9. Contemporary Developments

Modern systems increasingly use vacuum heat treatment, computer-controlled atmosphere control, rapid quenching, additive manufacturing repair, laser cladding, duplex treatments, and multilayer PVD coatings. Digital process monitoring can correlate furnace temperature, quench response, hardness, and distortion. Machine-learning tools may assist anomaly detection and parameter optimization, but they do not replace metallurgical validation, calibrated sensors, or destructive qualification tests.

Additively manufactured components may require stress relief, homogenization, solution treatment, aging, hot-isostatic pressing, or surface finishing because of porosity, anisotropy, and residual stress. The treatment must be developed for the specific alloy and manufacturing route rather than copied from wrought material practice.

Chapter Summary

Heat treatment controls the phase transformations and residual stresses that determine the bulk properties of metals. Annealing softens and relieves stress; normalizing refines structure; hardening produces martensite; and tempering restores toughness while retaining useful strength. Austempering and martempering provide alternatives when distortion and toughness must be balanced.

Case hardening creates a hard, wear-resistant surface over a tough core through carburizing, carbonitriding, nitriding, nitrocarburizing, induction hardening, or flame hardening. Surface engineering extends these principles through plating, conversion coatings, thermal spraying, vapour deposition, diffusion treatments, and mechanical modification such as shot peening.

Selection must be based on the actual failure mechanism, substrate, geometry, temperature, dimensional tolerance, environment, safety obligations, and life-cycle cost. Hardness, adhesion, residual stress, case depth, porosity, and distortion require systematic inspection. Successful engineering practice treats material, process, design, finishing, and quality assurance as an integrated system.

Key Terms

Heat treatment: Controlled heating, holding, and cooling used to change material structure and properties.

Austenite: Face-centred cubic phase of iron that dissolves relatively large amounts of carbon.

Ferrite: Low-carbon body-centred cubic phase of iron.

Pearlite: Lamellar mixture of ferrite and cementite.

Bainite: Fine ferrite–carbide transformation product formed at intermediate temperatures.

Martensite: Hard supersaturated phase formed by rapid, diffusionless transformation.

Hardenability: Capacity of a steel to harden to a given depth during quenching.

Quenching: Rapid cooling from an elevated temperature.

Tempering: Reheating quenched steel below the eutectoid temperature to improve toughness.

Annealing: Heat treatment generally involving slow cooling to soften or homogenize material.

Normalizing: Air cooling from the austenitizing range to refine structure and increase strength.

Case hardening: Production of a hard surface layer over a tougher core.

Carburizing: Diffusion of carbon into a steel surface.

Nitriding: Diffusion of nitrogen into a suitable alloy-steel surface.

Electroplating: Electrochemical deposition of a coating using external current.

Thermal spraying: Deposition of heated particles onto a prepared surface.

PVD: Vacuum deposition process using physical vaporization and ion assistance.

Shot peening: Bombardment that introduces beneficial compressive residual stress.

Retained austenite: Austenite remaining after quenching instead of transforming to martensite.

Effective case depth: Specified depth at which hardness or microstructure reaches a defined criterion.

Review Questions

5 Short Answer Questions

1. What is the difference between hardness and hardenability?
2. Why is tempering normally performed after hardening?
3. State two advantages and two limitations of nitriding.
4. How does shot peening improve fatigue performance?
5. Why can quenching produce distortion and cracking?

5 Long Answer Questions

1. Explain the phase transformations involved in annealing, normalizing, hardening, and tempering of carbon steel.
2. Compare carburizing, carbonitriding, nitriding, and induction hardening in terms of mechanism, case depth, distortion, and applications.
3. Describe the operating principles and limitations of electroplating, thermal spraying, PVD, and diffusion coatings.
4. Develop a quality-control plan for a heat-treated gear, including likely defects and appropriate inspections.
5. Discuss how material, geometry, quenching medium, and residual stress interact to determine component performance.

3 Analytical / Application Questions

1. A medium-carbon steel shaft requires a wear-resistant surface but must withstand torsional shock. Select a suitable treatment sequence and justify it using failure mechanisms and core–surface property requirements.
2. A carburized gear shows excessive distortion and retained austenite after quenching. Identify likely causes and recommend process or design changes.
3. A coated sliding component fails by delamination despite having high coating hardness. Analyze the possible roles of adhesion, substrate hardness, residual stress, lubrication, surface preparation, and contact loading.

Multiple Choice Questions with Answers

Question 1

Which transformation product is normally formed when austenite is cooled rapidly enough to suppress diffusional transformation?

A. Pearlite

B. Martensite

C. Spheroidized cementite

D. Proeutectoid ferrite

Correct Answer: B. Martensite

Question 2

What is the primary purpose of tempering quenched steel?

A. Increase carbon content at the surface

B. Convert ferrite into austenite

C. Reduce brittleness and residual stress while retaining useful hardness

D. Produce a zinc coating

Correct Answer: C. Reduce brittleness and residual stress while retaining useful hardness

Question 3

Which property describes the depth to which a steel can harden during quenching?

A. Surface roughness

B. Hardenability

C. Thermal conductivity

D. Machinability

Correct Answer: B. Hardenability

Question 4

Which process introduces nitrogen into a suitable alloy-steel surface, usually without a subsequent quench?

A. Nitriding

B. Full annealing

C. Normalizing

D. Copper plating

Correct Answer: A. Nitriding

Question 5

A gear requires a hard wear-resistant case and a tough shock-resistant core. Which treatment is most directly suited to this requirement?

A. Carburizing followed by quenching and tempering

B. Stress-relief annealing only

C. Full annealing only

D. Soft soldering

Correct Answer: A. Carburizing followed by quenching and tempering

Question 6

Which surface treatment primarily improves fatigue resistance by introducing compressive residual stress?

A. Shot peening

B. Decarburizing

C. Overheating

D. Acid pickling alone

Correct Answer: A. Shot peening

Question 7

What is a principal advantage of electroless nickel plating over conventional electroplating?

A. It always produces a thicker coating than thermal spraying

B. It can provide relatively uniform coverage on complex geometries

C. It requires no surface preparation

D. It eliminates all hydrogen-related risks

Correct Answer: B. It can provide relatively uniform coverage on complex geometries

Question 8

Which defect is especially associated with rapid cooling and excessive transformation or thermal stresses?

A. Quench cracking

B. Improved machinability

C. Increased ductility

D. Uniform anodizing

Correct Answer: A. Quench cracking

Question 9

Which process deposits heated or semi-molten particles onto a prepared substrate?

A. Thermal spraying

B. Spheroidizing

C. Austempering

D. Cold rolling

Correct Answer: A. Thermal spraying

Question 10

Why is a very hard coating not automatically a successful engineering solution?

A. Hardness has no relationship to wear

B. The coating may crack or delaminate if adhesion, substrate support, or residual stress is unsuitable

C. Hard coatings cannot be used on metals

D. Hard coatings always reduce corrosion resistance

Correct Answer: B. The coating may crack or delaminate if adhesion, substrate support, or residual stress is unsuitable