Material Properties and Heat Treatment
Introduction
This chapter covers the mechanical and physical properties of materials used in machining, as well as the principles and procedures of heat treatment. For the Red Seal exam, you must master the terminology, applicable Canadian standards, hardness and strength calculations, and heat treatment procedures. This chapter is designed to prepare you directly for typical exam questions.
2. Mechanical Properties of Materials
2.1 Fundamental Definitions
Mechanical properties describe how a material reacts under the application of forces. You must know these definitions precisely.
Tensile Strength: The maximum stress a material can withstand before rupture under tension. Expressed in MPa (megapascals) or ksi (kilopounds per square inch). The formula is σ = F / A, where σ is stress (MPa), F is force (N), and A is the cross-sectional area (mm²).
Yield Strength: The stress at which a material begins to deform plastically (permanent deformation). For mild steel, it is typically at 0.2% residual strain.
Ductility: The ability of a material to deform plastically before rupture. Measured by elongation (%) or reduction of area (%).
Toughness: The ability of a material to absorb energy before rupture. It combines strength and ductility. Measured by the Charpy test (energy absorbed in joules).
Hardness: Resistance to penetration or scratching. Measured by the Brinell (HB), Rockwell (HRC, HRB), Vickers (HV), and Shore (for elastomers) scales.
Fatigue: Progressive degradation under cyclic loads. The endurance limit is the maximum stress a material can withstand indefinitely without rupture (typically 50% of the tensile strength for steels).
Creep: Progressive deformation under constant load at high temperature (above 0.4 × absolute melting temperature).
2.2 Comparative Properties Table
| Material | Tensile Strength (MPa) | Yield Strength (MPa) | Hardness (HB) | Elongation (%) | Relative Machinability |
|---|
| 1018 Steel (mild) | 440 | 370 | 126 | 15 | 70% |
| 1045 Steel (medium) | 630 | 530 | 179 | 12 | 55% |
| 4140 Steel (alloy) | 1020 | 655 | 197 | 17 | 50% |
| Gray Cast Iron (class 30) | 207 | 130 | 180 | 0.5 | 100% |
| Aluminum 6061-T6 | 310 | 276 | 95 | 12 | 300% |
| Brass 360 (free-cutting) | 340 | 125 | 100 | 53 | 400% |
Exam Note: Relative machinability is based on AISI 1212 steel (100%). The higher the percentage, the easier the material is to machine.
2.3 Relationship Between Hardness and Strength
For carbon and alloy steels, there is an approximate correlation:
Tensile Strength (MPa) ≈ 3.45 × Brinell Hardness (HB)
Calculation Example: If a steel has a hardness of 200 HB, its approximate tensile strength is:
3.45 × 200 = 690 MPa
This relationship is valid for untreated or treated steels, but not for highly alloyed tool steels.
3. Classification of Steels According to Canadian Standards
3.1 AISI/SAE System
The AISI/SAE system uses a 4-digit code:
The first two digits indicate the type of steel:
10xx: carbon steels (non-alloyed)
11xx: carbon steels with high sulfur (free-machining)
12xx: carbon steels with high sulfur and phosphorus
13xx: manganese steels (1.75% Mn)
41xx: chromium-molybdenum steels (e.g., 4140)
43xx: nickel-chromium-molybdenum steels (e.g., 4340)
52xx: chromium steels (e.g., 52100 for bearings)
61xx: chromium-vanadium steels
86xx: nickel-chromium-molybdenum steels (e.g., 8620)
The last two digits indicate the carbon content in hundredths of a percent. For example, 1045 steel contains 0.45% carbon.
3.2 CSA and ASTM Standards
In Canada, structural steels are often specified according to CSA G40.20/G40.21 (weldable structural steels) and ASTM (American Society for Testing and Materials) standards. Tool steels are classified according to AISI (American Iron and Steel Institute).
Exam Point: The Canadian Electrical Code, Part I (CE Code) does not apply directly to machining materials, but machinists must know the grounding and safety requirements when working on electrical equipment. The CSA B149.1 (Natural Gas and Propane Installation Code) applies to gas installations, but machinists may encounter machined parts for these systems.
3.3 Tool Steels
Tool steels are classified into families:
W Series (water-hardening): low cost, shallow hardening depth.
O Series (oil-hardening): low distortion, good wear resistance.
A Series (air-hardening): low distortion, excellent dimensional stability.
D Series (high carbon and chromium): excellent wear resistance.
H Series (hot-work steels): for forging and extrusion dies.
T and M Series (high-speed steels): for high-speed cutting tools.
4. Heat Treatment of Steels
4.1 Iron-Carbon Diagram
The iron-carbon diagram is essential for understanding phase transformations. The critical points:
A₁ (723°C): Eutectoid temperature. Below this, the steel is completely ferritic and pearlitic.
A₃: The temperature at which ferrite completely transforms into austenite (for hypoeutectoid steels, < 0.8% C).
Acm: The temperature at which cementite dissolves into austenite (for hypereutectoid steels, > 0.8% C).
Eutectoid (0.8% C): The composition at which austenite transforms directly into pearlite.
Typical Compositions:
Hypoeutectoid steel: < 0.8% C (e.g., 1018, 1045)
Eutectoid steel: 0.8% C (e.g., 1080)
HyperEutectoid steel: > 0.8% C (e.g., 1095)
4.2 The Four Basic Heat Treatments
| Treatment | Heating | Cooling | Result |
|---|
| **Annealing** | Above A₃ (or A₁) | Slow (furnace off) | Softening, maximum ductility, coarse pearlitic structure |
| **Normalizing** | Above A₃ (or Acm) | Still air | Fine pearlitic structure, moderate hardness, homogenization |
| **Quenching** | Above A₃ (or Acm) | Rapid (water, oil, air) | Martensite, maximum hardness, brittleness |
| **Tempering** | Below A₁ (150–650°C) | Slow | Reduction of brittleness, adjustment of hardness/toughness |
4.3 Annealing
Purpose: To soften the steel, improve machinability, and eliminate internal stresses.
Procedure:
67.Heat to 30–50°C above A₃ (hypoeutectoid steels) or A₁ (hypereutectoid steels).
68.Hold at temperature (1 hour per 25 mm of thickness).
69.Cool slowly in the furnace (20–30°C/hour) down to 500°C, then air cool.
Result: Coarse pearlite + ferrite (hypoeutectoid) or pearlite + cementite (hypereutectoid). Typical hardness: 120–180 HB.
Exam Trap: Process annealing (or regulating annealing) is different from full annealing. Process annealing heats to only 20–40°C above A₁, then cools slowly. It is used for hypereutectoid steels to break up the cementite network.
4.4 Normalizing
Purpose: To refine the grain, homogenize the structure, and improve mechanical properties after forging or rolling.
Procedure:
75.Heat to 50–70°C above A₃ (or Acm).
76.Hold at temperature (1 hour per 25 mm).
77.Cool in still air (no drafts).
Result: Fine pearlite + ferrite (hypoeutectoid). Typical hardness: 180–220 HB. Normalizing produces a harder and stronger structure than annealing, but more ductile than quenching.
Practical Application: Normalizing is often performed before machining for medium-carbon steels (1045, 4140) to obtain a uniform structure and optimal machinability.
4.5 Quenching
Purpose: To obtain a martensitic structure, the hardest possible.
Procedure:
83.Heat to 30–50°C above A₃ (or Acm).
84.Hold at temperature to homogenize the austenite.
85.Cool rapidly in a quenching medium (water, oil, air, polymer).
Quenching Media:
| Medium | Cooling Rate | Application |
|---|
| Water + 10% salt | Very fast (500°C/s) | Simple carbon steels |
| Water | Fast (200°C/s) | Carbon steels |
| Oil | Moderate (100°C/s) | Alloy steels |
| Forced air | Slow (20°C/s) | High-hardenability alloy steels |
| Still air | Very slow | Tool steels (A series) |
Hardenability: The ability of a steel to harden in depth. Measured by the Jominy test (end quench). The test consists of quenching a 25 mm diameter cylindrical bar with a water jet on one end, then measuring hardness along the bar. The Jominy curve shows hardness as a function of distance from the quenched end.
Factors Affecting Hardenability:
Carbon content (increases maximum hardness)
Alloying elements (Mn, Cr, Mo, Ni, V — shift the TTT curve to the right)
Austenitic grain size (large grain = better hardenability)
Austenitizing temperature
4.6 Tempering
Purpose: To reduce the brittleness of martensite and adjust hardness and toughness.
Procedure:
97.Heat the quenched part to a temperature between 150°C and 650°C.
98.Hold at temperature (1 hour minimum, typically 2 hours).
99.Cool in air.
Effects of Tempering:
| Tempering Temperature (°C) | Resulting Hardness (HRC) | Structure | Application |
|---|
| 150–200 | 58–62 | Tempered martensite | Cutting tools, blades |
| 300–400 | 45–55 | Troostite | Springs, dies |
| 500–650 | 30–45 | Sorbite | Shafts, gears, connecting rods |
Exam Trap: Low-temperature tempering (150–200°C) reduces internal stresses without significantly decreasing hardness. High-temperature tempering (500–650°C) produces a good combination of strength and toughness but considerably reduces hardness.
4.7 Temper Embrittlement
Temper embrittlement occurs when certain alloy steels (particularly those containing chromium, manganese, or nickel) are cooled slowly after tempering between 375°C and 575°C. This embrittlement is reversible and can be avoided by:
Rapid cooling after tempering (in oil or water)
Adding molybdenum (0.2–0.5%) to the steel
5. Thermochemical Treatments
5.1 Carburizing
Purpose: To harden the surface of a low-carbon steel part (< 0.2% C) while maintaining a ductile core.
Procedure:
112.Heat the part to 870–950°C in a carbon-rich medium (solid, liquid, or gas).
113.Hold at temperature for 2 to 8 hours (case depth ≈ 0.1–1.5 mm).
114.Cool, then quench and temper.
Case Depth: The depth increases with time and temperature. Approximate rule: depth (mm) ≈ 0.025 × √(time in hours) at 925°C.
Application: Gears, camshafts, pinions, cylinder liners.
5.2 Nitriding
Purpose: To harden the surface by nitrogen diffusion at low temperature (500–550°C), without subsequent quenching.
Procedure:
120.Heat the part to 500–550°C in a dissociated ammonia atmosphere.
121.Hold at temperature for 20 to 80 hours.
122.Cool slowly.
Characteristics:
Very high surface hardness (1000–1200 HV)
Low distortion (no quenching)
Exceptional wear and fatigue resistance
Requires special steels (nitralloys, H13 tool steels)
5.3 Carbonitriding
A combination of carburizing and nitriding: simultaneous diffusion of carbon and nitrogen at 750–900°C. Produces a hard, wear-resistant layer with less distortion than conventional carburizing.
5.4 Surface Hardening (Induction and Flame)
Purpose: To harden only the surface of a medium-carbon steel part (0.35–0.55% C).
Procedure:
133.Heat the surface by electromagnetic induction or oxyacetylene flame.
134.Quench immediately by spraying with water or emulsion.
135.Temper at low temperature (150–200°C).
Application: Gears, shafts, crankshafts, cylinder liners.
6. Hardness Testing
6.1 Brinell Test (HB)
Tungsten carbide ball, 10 mm diameter.
Load: 3000 kgf for steels, 500 kgf for soft metals.
Measurement of the indentation diameter.
Formula: HB = 2F / (πD(D − √(D² − d²))), where F is the load (kgf), D is the ball diameter (mm), and d is the indentation diameter (mm).
Limitation: Not suitable for hardened surfaces (> 650 HB) or thin parts.
6.2 Rockwell Test (HRC, HRB, HRA)
Penetration by a diamond cone (120°) for HRC, or a 1/16 in steel ball for HRB.
Preliminary load: 10 kgf. Major load: 150 kgf (HRC), 100 kgf (HRB), 60 kgf (HRA).
Direct reading on the machine scale.
Common Scales:
HRC: 20–70 (quenched and tempered steels)
HRB: 20–100 (mild steels, aluminum, brass)
HRA: 60–85 (carbides, very hard materials)
6.3 Vickers Test (HV)
Diamond indenter, pyramidal shape (136° angle).
Loads: 1 to 120 kgf.
Formula: HV = 1.854 × F / d², where F is the load (kgf) and d is the average diagonal of the indentation (mm).
Advantage: Suitable for all materials, from softest to hardest.
6.4 Shore Test (HS)
Rebound of a diamond-tipped hammer on the surface.
Used for large components (rolling mill cylinders, large gears).
Less precise than other methods.
6.5 Approximate Conversion Table
| HRC | HB | HV | Tensile Strength (MPa) |
|---|
| 20 | 226 | 237 | 780 |
| 30 | 286 | 302 | 990 |
| 40 | 371 | 392 | 1280 |
| 50 | 481 | 513 | 1660 |
| 60 | 654 | 713 | 2260 |
Exam Trap: Conversions between hardness scales are approximate and are only valid for carbon and alloy steels. Do not use them for non-ferrous metals.
7. Mechanical Testing
7.1 Tensile Test
Standardized specimen (12.5 mm diameter, 50 mm gauge length per ASTM E8).
Stress-strain curve: elastic zone (Hooke's law), plastic zone, necking, rupture.
Modulus of Elasticity (E): Slope of the elastic zone. For steel: 200,000 MPa (207 GPa).
Tensile Strength: Maximum stress (point of rupture of the specimen).
Elongation (%): (Lf − L0) / L0 × 100, where Lf is the final length and L0 is the initial length.
7.2 Charpy Test (Impact Toughness)
V-notched specimen (10 × 10 × 55 mm).
Pendulum strikes the specimen opposite the notch.
Measures the energy absorbed (J) to fracture the specimen.
Ductile-to-Brittle Transition: Steels transition from ductile to brittle behavior with decreasing temperature. The transition temperature is typically between −20°C and −60°C for structural steels.
Typical Requirement: 27 J minimum at service temperature (per CSA G40.20).
7.3 Fatigue Test
Specimen subjected to cyclic loads (tension-compression, bending, torsion).
S-N curve (stress vs. number of cycles).
Endurance Limit: Maximum stress for 10⁷ cycles without rupture (steels). Aluminum alloys do not have a true endurance limit.
8. Material Identification
8.1 Spark Test
The spark test is a quick method for identifying carbon and alloy steels:
| Material | Spark Characteristics |
|---|
| Mild steel (1018) | Long yellow sparks, few branches |
| Medium-carbon steel (1045) | Shorter sparks, more numerous branches |
| Tool steel (W1) | Short sparks, star-shaped branches |
| Stainless steel (304) | Short, orange sparks, no branches |
| Gray cast iron | Very short, red, feather-shaped sparks |
8.2 File Test
A hard file cuts mild steel easily (HRB < 30).
A file slides over hardened steel (HRC > 55).
Qualitative but quick method.
8.3 Magnetism Test
Ferritic and martensitic steels are magnetic.
Austenitic steels (304, 316) are non-magnetic.
Aluminum, copper, and titanium alloys are non-magnetic.
9. Practical Applications in Machining
9.1 Material Selection by Application
| Application | Recommended Material | Heat Treatment |
|---|
| Transmission shaft | 4140 | Quench + temper (28–32 HRC) |
| Gear | 8620 | Carburizing + quench + temper |
| Cutting tool | M2 (high-speed steel) | Quench + multiple tempers |
| Forging die | H13 | Quench + temper (44–48 HRC) |
| Simple machine part | 1018 | Annealing or normalizing |
| Bearing | 52100 | Quench + temper (60–64 HRC) |
9.2 Machinability and Heat Treatment
Low-carbon steels (< 0.2% C): Excellent machinability in the annealed state, but gummy surface. Normalizing improves cutting.
Medium-carbon steels (0.3–0.5% C): Best machinability in the normalized or annealed state (160–200 HB).
Tool steels: Machinable in the annealed state (200–250 HB). After quenching, they can only be machined by grinding.
Gray cast iron: Excellent machinability, but abrasive (tool wear). Use coated carbides.
9.3 Distortion and Residual Stresses
Quenching causes dimensional distortions. To minimize:
Machine leaving a 0.5–1.0 mm allowance before heat treatment.
Perform a stress-relieving (stabilizing temper) at 150–200°C after rough machining.
After quenching and tempering, grind to obtain final dimensions.
For precision parts, plan a stabilization cycle (heating to 120°C, slow cooling) before final grinding.
10. Applicable Standards and Codes
10.1 Relevant Canadian Standards
CSA G40.20/G40.21: General requirements for weldable structural steels.
CSA W47.1: Certification of fusion welding of steel (general knowledge required for machinists working with welders).
CSA B149.1: Natural Gas and Propane Installation Code (for machined parts intended for gas systems).
Canadian Electrical Code, Part I (C22.1): Electrical safety requirements for machine tools (grounding, disconnection, lockout).
10.2 ASTM and ISO Standards
ASTM E8/E8M: Tension testing of metallic materials.
ASTM E18: Rockwell hardness testing.
ASTM E10: Brinell hardness testing.
ASTM E23: Charpy impact test.
ISO 683: Heat-treated steels, alloy steels, and free-cutting steels.
10.3 Important Safety Rule
According to the Canadian Electrical Code, Part I, Rule 8-200, every machine tool must be grounded and equipped with an accessible disconnecting means. Machinists must know these requirements to work safely.
11. Pitfalls to Avoid
228.Confusing annealing and normalizing: Annealing cools slowly in the furnace (coarse structure, minimum hardness). Normalizing cools in air (fine structure, moderate hardness). Do not mix them up.
229.Forgetting that quenching produces brittle martensite: Quenching alone is never sufficient for a functional part. Tempering is always necessary after quenching.
230.Using the hardness-strength conversion for non-ferrous metals: The relationship σ ≈ 3.45 × HB is only valid for steels. Do not apply it to aluminum or brass.
231.Ignoring the tempering temperature: A temper at 200°C and a temper at 500°C produce very different hardnesses. Always read the specification.
232.Heating a quenched steel above A₁: This cancels the quench and returns the structure to pearlite. Tempering must always be performed below 723°C.
233.Confusing carburizing and nitriding: Carburizing adds carbon at high temperature (with quenching). Nitriding adds nitrogen at low temperature (without quenching).
234.Neglecting hardenability: A simple carbon steel (1045) only hardens on the surface (5 mm max). For a thick part, an alloy steel (4140, 4340) is required.
235.Using the Brinell test on a hardened surface: The 10 mm ball is not suitable for surfaces > 650 HB. Use Rockwell C or Vickers.
236.Forgetting the machining allowance: A part that will be quenched must be machined with a 0.5–1.0 mm allowance to compensate for distortion.
237.Not knowing the 4-digit AISI codes: The last two digits indicate carbon in hundredths. 1045 = 0.45% C. 4140 = 0.40% C.
12. Summary
Key mechanical properties are tensile strength, yield strength, ductility, toughness, hardness, and fatigue resistance.
Brinell hardness (HB) is related to the tensile strength of steels by the approximate relationship: σ ≈ 3.45 × HB.
Steels are classified according to the 4-digit AISI/SAE system. The last two digits indicate carbon content in hundredths of a percent.
The four basic heat treatments are: annealing (softening), normalizing (grain refinement), quenching (hardening by martensite), and tempering (reducing brittleness).
Hardenability is measured by the Jominy test and depends on chemical composition and grain size.
Carburizing and nitriding are thermochemical treatments for hardening the surface while maintaining a ductile core.
Common hardness tests are Brinell (HB), Rockwell (HRC, HRB), Vickers (HV), and Shore (HS).
The Charpy test measures toughness and the ductile-to-brittle transition temperature.
Relevant Canadian standards include CSA G40.20/G40.21, CSA B149.1, and the Canadian Electrical Code, Part I.
For machining, plan a machining allowance before heat treatment and grind afterward for final dimensions.
13. Review Questions
252.What is the difference between yield strength and tensile strength?
253.How much carbon does 4140 steel contain? What are its main alloying elements?
254.Why is normalizing performed before machining a 1045 steel?
255.What is the critical temperature A₁ for carbon steels?
256.What is the effect of tempering at 600°C on a quenched steel?
257.Which hardness method would you use for a carburized surface 0.8 mm thick?
258.What is the difference between carburizing and nitriding?
259.How is hardenability measured?
260.What is the effect of molybdenum on temper embrittlement?
261.Which CSA standard applies to weldable structural steels?
14. Answers to Review Questions
264.Yield strength is the stress at which plastic deformation begins; tensile strength is the maximum stress before rupture.
265.0.40% carbon; chromium (0.8–1.1%) and molybdenum (0.15–0.25%).
266.To refine the grain, homogenize the structure, and improve machinability.
267.723°C (eutectoid temperature).
268.Reduces hardness (to about 30–35 HRC), improves toughness, produces a sorbite structure.
269.Vickers (HV) with a light load (1–10 kgf) to avoid penetrating the carburized layer.
270.Carburizing adds carbon at 870–950°C with quenching; nitriding adds nitrogen at 500–550°C without quenching.
271.By the Jominy test (end quench), which measures hardness as a function of distance from the quenched end.
272.Molybdenum (0.2–0.5%) eliminates or reduces temper embrittlement.
273.CSA G40.20/G40.21.
