Apply Heat Treatment and Metallurgical Processes

Applying Heat Treatments and Metallurgical Processes

Introduction to Heat Treatment of Steels

Heat treatment is the set of controlled heating and cooling operations applied to a metal in its solid state, with the goal of modifying its mechanical, physical, or chemical properties. For the tool and die maker, mastery of these processes is essential: dies, punches, cutting tools, and gauges must exhibit precise hardness, toughness, and wear resistance depending on their function.

The iron-carbon (Fe-C) diagram is the fundamental basis for understanding all heat treatments. It illustrates phase transformations as a function of temperature and carbon content (0 to 6.67% C). The important critical points are:

The notations Ac₁, Ac₃, Acm designate the critical temperatures on heating, while Ar₁, Ar₃, Arm designate those on cooling. This distinction is crucial: thermal hysteresis means that transformations on cooling occur at lower temperatures than those on heating.

The Four Basic Operations

1. Annealing

Annealing consists of heating the steel to a temperature above its critical point (generally 30 to 50 °C above A₃ or A₁), holding it at that temperature (soaking time), then cooling it very slowly (in the turned-off furnace or in an insulating medium such as lime, vermiculite, or ash).

Objectives:

Types of annealing:

Soaking time calculation: approximate rule of 1 hour per 25 mm (1 inch) of maximum part thickness, plus 1 additional hour. For alloy steels, this time must be increased by 25 to 50%.

2. Quenching

Quenching consists of heating the steel to the austenitizing temperature (generally A₃ + 30 to 50 °C), holding it at that temperature, then cooling it rapidly in an appropriate medium (water, oil, forced air, salt bath) to obtain a hard martensitic structure.

Typical austenitizing temperatures:

Quenching media and their severity:

Risks of quenching:

Precautions: preheating at 500–650 °C for alloy steels, protection with a controlled atmosphere or stainless steel wrap, immersion with agitation, correct part orientation (thin sections enter first).

3. Tempering

Tempering is a mandatory heat treatment after quenching. It consists of reheating the quenched steel to a temperature below A₁ (150–650 °C), holding it at that temperature, then cooling it in still air.

Objectives:

Effect of tempering temperature on hardness (quenched 0.80% C steel):

Temper embrittlement: certain alloy steels (Cr-Ni, Cr-Mn) exhibit increased brittleness when cooled slowly through the 375–575 °C zone after tempering. The solution is to cool rapidly (in oil or water) after tempering in this range.

Practical rule: tempering soak time is 1 hour per 25 mm of thickness, with a minimum of 1 hour.

4. Normalizing

Normalizing consists of heating the steel to A₃ + 50–60 °C, holding it, then cooling it in still air. It refines the grain and homogenizes the structure. It is often applied before machining or before quenching for forged or rolled parts.

Thermochemical Treatments

Carburizing

Carburizing is a process that introduces carbon into the surface layer of a part made of mild or low-alloy steel (0.10–0.20% C), in order to obtain a hard surface (58–62 HRC) and a tough, strong core.

Methods:

Case depth: depends on time and temperature. Approximate rule: depth (mm) ≈ 0.025 × √(time in hours) at 925 °C. For example, 4 hours gives approximately 0.5 mm.

After carburizing: the part is quenched (directly or after cooling and reheating) then tempered at low temperature (150–200 °C).

Nitriding

Nitriding introduces nitrogen into the steel surface at a temperature of 500–580 °C, without austenitic transformation. It produces a very hard layer (68–72 HRC) with excellent wear and fatigue resistance, but a shallow depth (0.1–0.5 mm).

Suitable steels: special steels containing aluminum, chromium, molybdenum, or vanadium (e.g., Nitralloy 135M, H13).

Advantages: no distortion (low temperature), no subsequent treatment required, improved corrosion resistance.

Carbonitriding and Nitrocarburizing

Carbonitriding combines carbon and nitrogen at 750–900 °C, producing a hard case with increased hardenability. Nitrocarburizing (or ferritic carbonitriding) is done at 570–590 °C and produces a thin layer (10–20 μm) with very high wear resistance.

Surface Treatments

Cryogenic Treatment

Cryogenic treatment (cooling to -80 °C or -196 °C in liquid nitrogen) is applied after quenching and before tempering to convert retained austenite into martensite. It improves dimensional stability and tool life for cutting tools.

Ion Nitriding (Plasma)

Performed under vacuum with a nitrogen plasma at 350–550 °C, it allows precise control of case thickness and avoids the brittle white layer. Used for extrusion dies and injection molds.

Hardness Testing

Hardness verification is essential after any heat treatment. The most common scales:

Approximate conversion rule: HRC ≈ (HB/10) – 15 for steels in the 200–400 HB range. These conversions are approximate and must not replace a direct test.

Precautions during Rockwell testing: clean and flat surface, minimum thickness of 10 times the penetration depth, distance between indentations of at least 3 indentation diameters, rigid and clean support.

Tool Steels: Classification and Treatment

Tool steels are classified according to the AISI/SAE system:

Example of a complete cycle for O1 steel (oil-hardening):

Double tempering: mandatory for high-speed steels (M2, T1) and recommended for hot-work steels (H13). The first temper transforms retained austenite into martensite; the second temper tempers this new martensite.

Heat Treatment Defects and Their Prevention

Furnaces and Equipment

Furnaces used in toolmaking are classified according to their energy source and mode of operation:

Temperature control: thermocouples (type K: chromel-alumel, type S: platinum-rhodium) must be calibrated regularly. The typical tolerance is ±5 °C for tooling treatments.

Practical Calculations and Golden Rules

Heating time: rule of 1 minute per millimeter of thickness for carbon steels, 2 minutes per millimeter for alloy steels, starting from the preheat temperature.

Salt bath dilution volume: to adjust a bath temperature, add cold salt in small quantities (never more than 5% of the total volume at once).

Case depth: d (mm) = K × √t, where K ≈ 0.5 at 925 °C, 0.4 at 900 °C, 0.3 at 875 °C, and t in hours.

Hardness after tempering: hardness decreases by approximately 1 HRC per 10 °C increase in tempering temperature in the 200–400 °C range for carbon steels.

Applicable Canadian Standards

Although heat treatment is primarily governed by internal specifications and ASTM/SAE standards, the tool and die maker must be familiar with the following Canadian standards:

Compliance with occupational health and safety regulations (WHMIS for quenching salts and atmospheres) is also mandatory.

Pitfalls to Avoid

Summary

To pass the exam: memorize the temperature ranges for each treatment, the quenching media suited to each type of steel, and the soak time rules. Practice reading a complete heat treatment cycle and identifying errors in a given sequence.