Select and Prepare Materials and Tool Steels

Selecting and Preparing Materials and Tool Steels

Introduction to the Role of Material in Tooling

Material selection is the first critical technical decision in the manufacture of a tool, die, or mold. A tool can be perfectly designed and machined, but if the material is poorly selected or improperly prepared, it will fail prematurely through fracture, deformation, wear, or thermal fatigue. For the Red Seal exam, you must demonstrate a thorough understanding of tool steel families, their heat treatments, and selection criteria based on the final application.

This chapter covers classification systems (AISI/SAE and CSA), relevant mechanical and physical properties, heat treatment procedures (annealing, hardening, tempering), surface treatments, and hardness verification methods. Particular attention is given to applicable Canadian standards and common pitfalls identified in exams.

Steel Classification Systems

The AISI/SAE System for Tool Steels

In Canada, tool steel classification primarily follows the American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) system. This system uses an alphabetic prefix indicating the family, followed by a number. The main families are:

Mnemonics for the exam: the letters often correspond to the quenching method or primary use. "W" for Water, "O" for Oil, "A" for Air, "D" for Die, "H" for Hot work, "T" and "M" for high-speed steels (Tungsten and Molybdenum).

The CSA System for Steels

CSA Group (Canadian Standards Association) publishes standards relating to steels, notably CSA G40.20 and CSA G40.21 for structural steels, and CSA W47.1 for welding process qualification. Although these standards primarily concern structural steels, you should know that tool steels are generally specified by AISI/SAE designations, and that CSA standards apply to support materials, mounting plates, and tooling structures.

Exam point: Standard CSA G40.20-13/G40.21-13 covers general requirements for hot-rolled, welded, and cold-formed structural steels. It is often cited in questions about base materials for mounting plates and tooling bases.

Material Properties Relevant to Tooling

Hardness

Hardness is a material's resistance to penetration or localized plastic deformation. It is the most commonly specified property for tool steels. The main scales are:

Approximate conversion formula: for tool steels, HRC ≈ (HB / 10) - 10, but this conversion is only valid within a limited range. The exam may ask you to interpret a conversion table, but not to calculate manually.

Toughness

Toughness is a material's ability to absorb energy before fracture. It is measured by the Charpy impact test (notched specimen impact test). Tool steels must balance hardness and toughness: a steel that is too hard will be brittle, while a steel that is too tough will wear quickly.

Wear Resistance

Wear resistance depends primarily on carbon content and carbides (chromium, vanadium, tungsten, molybdenum). Hard carbides in the martensitic matrix act as barriers to abrasion. D2 steel, with 1.5-2.3% carbon and 11-13% chromium, offers excellent wear resistance but limited toughness.

Hot Hardness

Hot hardness (or red hardness) is the ability to retain hardness at elevated temperatures. Hot work steels (H13) and high-speed steels (M2, T1) retain their hardness up to 500-600 °C, making them essential for forging dies and high-speed cutting tools.

Machinability

Machinability is the ease with which a material can be cut. Tool steels are generally supplied in the annealed condition (200-250 HB hardness) to facilitate machining. The addition of sulfur ("free-machining" steels) can be specified, but this reduces toughness.

Tool Steel Selection by Application

Selection Criteria

Choosing a tool steel requires a systematic analysis of the following requirements:

Quick Selection Table

Golden rule for the exam: if the question mentions a service temperature above 200 °C, immediately eliminate cold work steels (W, O, A, D) and choose a hot work steel (H) or high-speed steel (M, T).

Heat Treatment of Tool Steels

The Iron-Carbon Diagram and Phase Transformations

You must master the critical points of the iron-carbon diagram:

For tool steels (carbon content from 0.5 to 2.3%), most are hypereutectoid (more than 0.77% C). The presence of primary and secondary carbides directly influences wear resistance.

Annealing

Annealing is a heat treatment aimed at softening the steel, improving machinability, and preparing the microstructure for hardening. For tool steels, full annealing or spheroidizing annealing is mainly used:

Objective: obtain a pearlitic or spheroidized structure with a hardness of 200-250 HB.

Exam trap: annealing is not the same as normalizing. Normalizing cools in air, producing a harder structure (fine pearlite) and is not appropriate for highly alloyed tool steels.

Hardening (Quenching)

Quenching consists of heating the steel above the critical temperature (A3 or Acm) to form austenite, then cooling rapidly to form martensite, a hard and brittle structure.

Typical procedure for O1 steel:

Quenching temperatures for common steels:

Holding time rule: the soak time at temperature is generally 1 minute per millimeter of thickness, with a minimum of 10 minutes for small parts.

Tempering

Tempering is a mandatory heat treatment after quenching. It consists of reheating the hardened steel to a temperature below A1 (150-650 °C) to:

Effect of tempering temperature on hardness (O1 steel):

Temper embrittlement: certain steels (particularly those containing manganese, chromium, or nickel) exhibit temper embrittlement between 250 and 400 °C. This range should be avoided, or the steel should be cooled rapidly after tempering.

Multiple tempering: for hot work steels (H13) and high-speed steels (M2), double or triple tempering is necessary to transform retained austenite into tempered martensite. Each tempering cycle lasts 1-2 hours.

Retained Austenite

During quenching, some austenite may not transform into martensite. This retained austenite is soft and unstable. For high-alloy steels (D2, M2), it can represent 20-30% of the microstructure.

Solutions:

Exam trap: cryogenic treatment must be performed immediately after quenching, before the first tempering cycle, to be effective.

Surface Treatments and Coatings

Nitriding

Nitriding is a thermochemical treatment that introduces nitrogen into the steel surface at a temperature of 500-580 °C, without phase transformation. It produces a very hard surface layer (1000-1200 HV) with excellent wear and fatigue resistance.

Application: forging dies (H13), injection molds, machine components.

Carburizing

Carburizing is a thermochemical treatment that introduces carbon into the surface of a low-carbon steel (< 0.3% C) to create a hard case (58-62 HRC) over a tough core.

Note: carburizing is not used for tool steels (already rich in carbon), but for tooling mechanism parts (pillars, columns, bushings).

PVD and CVD Coatings

Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) coatings significantly improve the service life of cutting tools:

Exam point: TiAlN coating is preferred for high-speed dry machining because it forms a protective alumina layer at high temperatures.

Hardness Testing and Verification

Rockwell Test

The Rockwell test is the most common in tool and die shops. It measures penetration depth under load.

Precautions:

Brinell Test

The Brinell test uses a 10 mm tungsten carbide ball with a 3000 kgf load for steels. The indentation diameter is measured and converted to an HB number.

Formula: HB = 2F / (πD√(D² - d²)), where F is the load in kgf, D is the ball diameter in mm, and d is the indentation diameter in mm.

Application: inspection of raw parts, annealed steels, large surfaces.

Vickers Test

The Vickers test uses a square-based diamond pyramid with loads from 1 to 120 kgf. It is particularly suited for thin layers and microhardness measurements.

Formula: HV = 1.854 × F / d², where F is the load in kgf and d is the average of the indentation diagonals in mm.

Applicable Canadian Standards

CSA W47.1 — Welding Qualification

Standard CSA W47.1 (Certification of Companies for Fusion Welding of Steel) applies to the welding of structural steels and tool steels. For tool and die makers, this standard is relevant when repairing dies by welding. Key requirements include:

Practical rule: for welding H13 steel, a preheat of 300-350 °C and immediate post-heat at 550-600 °C are mandatory to prevent cracking.

CSA G40.20/G40.21 — Structural Steels

Standard CSA G40.20-13/G40.21-13 specifies requirements for hot-rolled structural steels. Common grades include:

These steels are used for bases, mounting plates, and load-bearing tooling structures.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1) applies to electrical installations of tooling equipment, including induction heating systems, heat treatment furnaces, and machine controls. Rule 8-200 concerns the calculation of electrical demand for motors and heating equipment.

Exam point: heat treatment furnaces must comply with the requirements of CSA C22.2 No. 88 for industrial heating appliances.

Material Preparation Procedures

Receiving and Identification

Upon receiving a tool steel, you should know how to:

Decarburization

Decarburization is the loss of carbon from the steel surface during heating. It occurs when steel is heated above 700 °C in an oxidizing atmosphere. The decarburized layer is softer and must be removed by machining or grinding before quenching.

Prevention:

Exam trap: decarburization lowers surface hardness by 2 to 5 HRC. A hardness check on a decarburized part will give falsely low readings.

Machining Before Heat Treatment

General rules for machining tool steels:

Distortion During Quenching

Distortion during quenching is caused by:

Minimization:

Practical Calculations for the Tool and Die Maker

Grinding Allowance Calculation

If a part must have a hardness of 58-60 HRC after quenching and tempering, and the estimated distortion is 0.05 mm, the minimum grinding allowance is:

Allowance = Estimated distortion + 0.2 mm (minimum to remove decarburization)

Example: distortion of 0.05 mm → allowance = 0.05 + 0.2 = 0.25 mm per surface.

Furnace Load Calculation

For a heat treatment furnace, the maximum load is determined by the furnace power and the mass of the parts:

Heating time (hours) = (Total mass in kg) / (Furnace power in kW × 0.5)

This formula is approximate and depends on the geometry of the parts.

Thermal Expansion Calculation

The thermal expansion of tool steels is approximately 11-13 × 10⁻⁶ /°C. For a 200 mm part heated from 20 °C to 800 °C:

ΔL = L₀ × α × ΔT = 200 mm × 12 × 10⁻⁶ /°C × 780 °C = 1.87 mm

Application: this calculation is essential for precision fixtures and gauge blocks.

Quality Control and Documentation

Heat Treatment Certificates

Each heat treatment cycle must be documented with:

Hardness Inspection After Treatment

Hardness inspection should be performed:

Summary

Pitfalls to Avoid