HARDOX500 vs HARDOX600 – Composition, Heat Treatment, Properties, and Applications

Introduction

HARDOX 500 and HARDOX 600 are quenched-and-tempered, wear-resistant structural steels widely used in heavy-duty applications where surface wear is the primary design driver. Engineers, procurement specialists, and manufacturing planners frequently compare these grades when balancing wear life, structural performance, welding and fabrication constraints, and overall cost.

The core distinction between the two is the trade-off between higher nominal hardness (and thus higher resistance to abrasive wear) versus retained toughness and greater ease of fabrication. HARDOX 600 is engineered to provide a higher nominal hardness class than HARDOX 500, while HARDOX 500 offers a more balanced combination of strength and toughness for many welded fabrications. These differences make them complementary rather than interchangeable for every application.

1. Standards and Designations

• Common commercial designation: HARDOX (product name, SSAB).
• Typical standards and normative frameworks where you may specify or test plates:
• EN (European Norms) — where supplier can deliver to customer-specified mechanical and chemical requirements.
• ASTM / ASME — for general structural applications; although HARDOX is a proprietary grade, material certificates and test methods referenced to ASTM standards are commonly supplied.
• JIS / GB — regional standards used in Asia; HARDOX plates are often supplied with certificates traceable to local testing standards.
• Material classification: High-strength, quenched-and-tempered low-alloy steel (not stainless, not tool steel, sometimes classed as HSLA with heat treatment to achieve very high hardness and strength).

2. Chemical Composition and Alloying Strategy

Manufacturers publish nominal chemistries for their wear-steel families; exact formulations and heat-treatment schedules are proprietary. Instead of absolute elemental numbers, the table below summarizes typical alloying roles and relative presence for HARDOX 500 and HARDOX 600.

Explanation: HARDOX steels rely on a controlled carbon content combined with manganese and small additions of hardenability elements (Cr, Mo, sometimes Ni and microalloying elements) to achieve a martensitic or bainitic-martensitic structure after quenching and tempering. Higher nominal hardness in HARDOX 600 is achieved by alloy and heat-treatment adjustments that increase hardenability and martensitic stability; these adjustments tend to reduce ductility and require stricter welding/fabrication controls.

3. Microstructure and Heat Treatment Response

• Typical microstructure (as-delivered): Predominantly tempered martensite with a finely dispersed population of carbide and microalloy precipitates. The microstructure is refined by controlled rolling and quenching to produce high hardness while retaining some toughness.
• HARDOX 500: Heat-treatment and thermo-mechanical rolling are tailored to produce a hard martensitic matrix with relatively good impact toughness for the given hardness. The plate is typically quenched from high temperature and tempered to control hardness and toughness.
• HARDOX 600: Processed to produce a harder tempered martensite with higher carbon/enhanced hardenability elements, resulting in a higher fraction of martensite and a lower fraction of retained austenite after tempering. This yields greater abrasive resistance but lower elongation and reduced impact toughness.
• Influence of processing:
• Normalizing: Raises toughness uniformly but does not achieve the high hardness required for these grades.
• Quench & temper: Primary processing route; quenching produces martensite, tempering reduces brittleness and adjusts hardness/toughness balance.
• Thermo-mechanical rolling (TMCP): Helps produce a finer ferritic/pearlitic prior structure before quench, improving toughness and uniformity; commonly used for HARDOX production.

4. Mechanical Properties

Below is a comparative, application-oriented summary rather than absolute guaranteed numbers (consult supplier certificates for project-specified values).

Why: HARDOX 600 is produced to a higher hardness class, giving it superior resistance to abrasive wear and higher static strength. That higher hardness is achieved by increasing hardenability (alloying and heat treatment), which also decreases ductility and impact energy absorption compared with HARDOX 500.

5. Weldability

Key weldability drivers: carbon content, effective hardenability, and presence of microalloying elements.

Useful empirical indices (interpret qualitatively in this context):

• Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm (more conservatively predicts cold cracking susceptibility): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - Both HARDOX grades have relatively low absolute carbon compared with tool steels, but their hardenability is increased by Mn, Cr, Mo and microalloying. Consequently, predicted $CE_{IIW}$ and $P_{cm}$ values trend upward for HARDOX 600 versus HARDOX 500. - Practical welding implications: - Preheat and interpass temperatures: HARDOX 600 generally requires higher preheat and more controlled interpass temperature to avoid cold cracking and to control HAZ hardness. - Filler selection: Use compatible weld consumables designed to match or slightly undermatch strength and to produce a tougher weld metal. - Post-weld heat treatment (PWHT): Often not applied for large fabrications; instead, control of heat input, preheat, and use of multi-pass welding strategies is preferred. - Hardness in HAZ: Beware of hard and brittle HAZ if welding without adequate preheat—more pronounced in HARDOX 600. - Practical guidance: Both grades can be welded successfully with standard industry procedures, but HARDOX 600 requires stricter procedures, more conservative travel speeds or preheat, and validated welding procedure qualification records (WPS/PQR).

6. Corrosion and Surface Protection

• Neither HARDOX 500 nor HARDOX 600 is stainless; they are carbon/alloy steels designed for wear resistance, not corrosion resistance.
• Typical surface protection strategies:
• Painting and industrial coatings (epoxy, polyurethane) for atmospheric protection.
• Thermally sprayed coatings for combined wear and corrosion environments.
• Galvanizing is possible for parts where form and function permit, but pre- and post-processing must account for plate thickness and heat treatment effects.
• PREN (Pitting Resistance Equivalent Number) is not applicable because PREN applies to stainless alloys. For reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Clarification: Use of sacrificial coatings or inhibitors is the norm for HARDOX applications where corrosion is a concern; corrosion resistance should not be assumed.

7. Fabrication, Machinability, and Formability

• Cutting: Plasma and oxy-fuel cutting common; laser cutting feasible but requires parameter adjustment. Hardness impacts consumable life and cutting speeds.
• Machining: Both are harder to machine than mild steel; HARDOX 600 is more demanding due to higher hardness—expect lower cutting speeds, more robust tooling, and potentially cryogenic or carbide tooling strategies.
• Bending/forming: Cold forming is limited by hardness—springback and cracking risk increase with HARDOX 600. Bending radii and tooling must be conservatively selected; preheating or hot-forming strategies are sometimes used for complex shapes.
• Surface finishing: Grinding and surface preparation take longer on HARDOX 600; abrasive wheel wear is greater.
• Handling: Higher hardness implies higher brittleness potential at stress concentrators—edge preparation and chamfering are more important on HARDOX 600.

8. Typical Applications

Selection rationale: Choose HARDOX 500 when design requires a balance of toughness, weldability and abrasion resistance (typical for mobile and welded structures). Choose HARDOX 600 when maximized abrasion resistance is the governing design criterion and fabrication constraints can be managed.

9. Cost and Availability

• Relative cost: HARDOX 600 is typically more expensive per kilogram or square meter than HARDOX 500 due to increased alloying, tighter processing, and lower production volumes.
• Availability: HARDOX 500 generally has broader availability in a wider range of plate thicknesses and finishes. HARDOX 600 availability is good for common plate sizes but may be limited in very thick plates or niche thickness/width combinations.
• Product forms: Sheets and plates, wear liners, pre-formed parts. Long lead times can occur for custom sizes or surface treatments.

10. Summary and Recommendation

Choose HARDOX 500 if: - Your application requires a balance between wear resistance and toughness (e.g., mobile equipment bodies, large welded structures). - Welding, forming, or bending operations are frequent and cost-sensitive. - You require wider availability and lower material cost per part.

Choose HARDOX 600 if: - Abrasion is the dominant failure mode and maximizing wear life justifies higher material and fabrication controls (e.g., severe mining, primary crushing surfaces). - The part geometry is simple or pre-fabricated wear components are used to avoid complex forming/welding of very hard plates. - You are prepared to follow stricter welding procedures and possibly accept higher replacement/repair costs to gain longer in-service lifetime.

Final note: HARDOX is a family of proprietary, high-performance quenched-and-tempered steels. For design, fabrication, and procurement, always consult the supplier’s current datasheets and certificates for exact chemical compositions, guaranteed mechanical properties, recommended welding procedures, and handling instructions. Field testing—trial inserts or pilot runs—can be the most reliable way to validate grade selection for a specific abrasive and impact environment.