AR400 vs AR450 – Composition, Heat Treatment, Properties, and Applications

Introduction

Abrasion-resistant (AR) steels AR400 and AR450 are two of the most commonly specified quenched-and-tempered wear steels for components subjected to sliding, impact, and abrasive contact. Engineers, procurement managers, and manufacturing planners frequently weigh trade-offs such as cost versus service life, weldability versus hardness, and toughness versus wear resistance when choosing between them. Typical decision contexts include choosing wear liners, bucket edges, crusher parts, or AR plate for fabrication where service loading and impact severity differ.

The primary practical distinction between AR400 and AR450 is their target hardness level and the consequent balance of strength and toughness. This difference drives choice in applications: AR400 is usually chosen where higher toughness and formability are needed, and AR450 where increased hardness and abrasion resistance justify somewhat reduced ductility and potentially more conservative welding and forming practices. Because both are commercial designations rather than single harmonized material standards, they are frequently compared side-by-side in design and procurement.

1. Standards and Designations

• AR400 and AR450 are commercial abrasion-resistant (AR) steel grades used internationally. They are typically supplied as quenched-and-tempered plates.
• They are not single ASTM chemical-designation grades; instead they are trade names used by multiple producers. Comparable branded products include Hardox 400/450 (SSAB) and other manufacturer-specific AR grades.
• For related specifications, engineers often consult standards that cover quenched-and-tempered structural/alloy steels (examples: certain ASTM specifications for high‑strength quench and tempered plates) or national standards for wear-resisting steels.
• Classification: AR400 and AR450 are alloyed, quenched-and-tempered carbon/alloy steels intended chiefly for wear resistance (they are not stainless steels).

2. Chemical Composition and Alloying Strategy

Manufacturers supply AR400 and AR450 with varying chemistries. The alloying approach is to provide enough hardenability and tempering response to achieve target hardness while retaining acceptable toughness and weldability. Rather than fixed percentages (which vary by supplier and plate thickness), the following table summarizes the typical role and relative presence of the listed elements in commercial AR400/AR450 products.

How alloying affects properties: - Hardenability: Carbon, Mn, Cr, Mo, and B primarily determine the ease of forming martensite on quenching; higher hardenability supports higher hardness in thicker plates. - Strength vs. Toughness: Carbon and alloying increase hardness/strength but can reduce toughness; microalloying (V, Nb, Ti) refines grain size and helps toughness. - Corrosion: These are not corrosion-resistant alloys—chromium content is generally too low for stainless behavior.

3. Microstructure and Heat Treatment Response

Typical microstructures - Both AR400 and AR450 achieve their wear properties via a martensitic or tempered martensitic microstructure produced by quenching and tempering. The as-quenched structure is predominantly martensite; tempering reduces brittleness, improves toughness, and controls final hardness. - AR400: Target hardness is lower than AR450, so tempering temperatures or quench severity are adjusted to produce slightly softer tempered martensite with relatively higher retained toughness and ductility. - AR450: Target hardness is higher, meaning the heat-treatment aims for a tougher martensitic matrix with lower tempering or higher quench severity, which can reduce ductility and impact toughness relative to AR400.

Effects of processing routes - Normalizing: Typically used as a pre-treatment for homogenization and grain refinement; AR steels intended for quench-and-temper service benefit from controlled austenitizing prior to quenching. - Quenching & tempering: The dominant route. Quench hardness is controlled by austenitizing temperature, hold time, quench medium, and subsequent tempering temperature/time to achieve target Brinell hardness. - Thermo-mechanical processing: Some suppliers use controlled rolling and accelerated cooling to refine microstructure and reduce alloy content while maintaining hardness/toughness balance.

4. Mechanical Properties

Because mechanical properties are influenced by manufacturer, plate thickness, and heat treatment, present comparisons are qualitative and focused on relative behavior. The defining numeric distinction is hardness: AR400 denotes an approximate nominal hardness near 400 HB, and AR450 near 450 HB.

Explanation: - AR450 achieves higher hardness and therefore generally higher tensile/yield strength, making it superior for abrasive wear resistance. - The harder AR450 typically sacrifices some ductility and impact toughness compared to AR400. For components exposed to high-impact or heavy shock loading, AR400 is often the safer choice to avoid brittle fracture.

5. Weldability

Weldability of AR plate depends on chemical composition (especially carbon and equivalent elements), hardenability, and the propensity to form hard martensitic heat-affected zones (HAZ) during welding.

Useful empirical indices: - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (weldability parameter): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Higher carbon and alloy content increase $CE_{IIW}$ and $P_{cm}$, indicating a higher risk of HAZ hardening, cracking sensitivity, and preheating needs. - AR400 is generally easier to weld than AR450 because of its lower target hardness and typically lower hardenability requirement, but both grades can require preheat, interpass temperature control, and post-weld heat treatment depending on thickness and chemistry. - Best practice: consult supplier weld procedures, use low-hydrogen electrodes, control preheat/interpass temperatures, and consider post-weld tempering to restore toughness in the HAZ.

6. Corrosion and Surface Protection

• AR400 and AR450 are not stainless steels; corrosion resistance is limited and typically similar to mild steel unless a specific alloying variant includes elevated chromium.
• Common surface protection strategies:
• Painting/coating (epoxy or polyurethane systems) for atmospheric protection.
• Hot-dip galvanizing is feasible on some AR steels but may require acceptance testing because heat treatment can affect properties; consult supplier.
• Local sacrificial wear coatings (wire-clad overlays, weld overlays) are sometimes applied to extend life.
• PREN is not applicable because these are not stainless grades; for stainless grades the index would be: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

7. Fabrication, Machinability, and Formability

• Formability and bending: AR400, being softer/ tougher, bends and forms more readily than AR450. As hardness increases, springback and the risk of cracking at bend radii increase.
• Cutting and machining: Higher hardness (AR450) increases tool wear and may require carbide tooling and slower feeds. Plasma, laser, and waterjet cutting are commonly used; cutting parameters must be optimized for hardness and thickness.
• Finishing: Grinding and polishing are more labor-intensive on AR450 because of higher abrasive wear resistance—grading and surface preparation for coatings can also require more aggressive methods.
• Fabrication recommendations: Use pre-qualified procedures; minimize cold work after heat treatment; select tool materials and processes appropriate for hardness.

8. Typical Applications

Selection rationale: - Choose AR400 when service includes frequent impacts, shock loads, or when forming/welding operations demand greater ductility and toughness. - Choose AR450 when abrasion is the dominant failure mode and higher hardness will materially extend maintenance intervals, and where welding/forming challenges can be managed.

9. Cost and Availability

• Relative cost: AR450 typically costs more per ton/plate than AR400 due to the additional processing and/or alloying needed to reach a higher hardness specification.
• Availability: Both grades are widely available in common plate sizes and thicknesses; availability of exact thickness/length combinations can vary by mill and region. Hardness-targeted branded products (e.g., Hardox) may have more consistent supply in some markets.
• Product forms: Standard supply is quenched-and-tempered plate; some suppliers also offer fabricated wear parts, liners, and profiles.

10. Summary and Recommendation

Recommendations: - Choose AR400 if your component requires a balance of wear resistance and impact/toughness, you anticipate field welding or forming, or you need better resistance to brittle fracture. - Choose AR450 if your primary failure mode is abrasive wear, where the extra hardness will substantially increase service life, and you can accept tighter welding/forming controls and potentially higher initial material cost.

Final note: Because AR grades are supplied with varying chemistries and heat‑treatment practices, always obtain supplier material certificates, recommended welding procedures, and hardness verification for the specific plate thickness and heat-treatment state before approving material for critical service.