NM550 vs HARDOX550 – Composition, Heat Treatment, Properties, and Applications

Introduction

NM550 and HARDOX550 are two widely compared wear-resistant steel grades used across mining, quarrying, heavy equipment, earth-moving, and material handling industries. Engineers, procurement managers, and manufacturing planners commonly choose between them when specifying wear plates or structural components exposed to abrasive service. Typical decision contexts include balancing wear life versus purchase cost, optimizing weldability and fabricability versus delivered hardness and toughness, and choosing between proprietary certified supply chains versus locally available alternatives.

The principal distinction between the two lies in how their chemistry, processing, and quality control are combined to deliver hardness, toughness, and predictable performance at the high end of the 550 hardness class. HARDOX550 is a commercial, quenched-and-tempered, proprietary product with tightly controlled processing to achieve a consistent combination of high hardness and verified toughness; NM550 is a non-proprietary / market grade designation used in certain regions and represents steels manufactured to meet a 550-class hardness target but with more variable alloying and production practices. This is why they are commonly compared: both target the same nominal hardness level, yet deliver different guarantees on microstructural uniformity, toughness, and certified properties.

1. Standards and Designations

• Major international standards and systems that are relevant when specifying wear steels include: ASTM/ASME, EN (European Norms), JIS (Japanese Industrial Standards), and GB (Chinese national standards). Additionally, proprietary mill brands (e.g., HARDOX by SSAB) have their own quality systems and delivery conditions.
• Classification by steel family:
• NM550: Typically categorized as a high-hardness wear-resistant steel within the general category of quenched and tempered high-strength low-alloy (HSLA) steels / wear steels. It is often identified by regional standards and commercial specifications rather than a single international standard.
• HARDOX550: A branded quenched-and-tempered wear-resistant steel supplied by SSAB. It is a wear steel with properties engineered and guaranteed by the manufacturer; technically it is an alloyed, heat-treated structural steel intended for demanding abrasive applications.

2. Chemical Composition and Alloying Strategy

The exact alloying of these steels can vary by producer. Instead of specific numeric values, the table below summarizes the typical roles and relative presence of common elements in wear-resistant steels of the 550-hardness class.

How alloying affects performance: - Carbon and alloying elements (Cr, Mo, Mn, etc.) increase hardenability and enable achieving a high hardness after heat treatment, but higher carbon and alloy content tends to reduce weldability and can impair impact toughness if not balanced. - Microalloying elements (V, Nb) are used to refine prior austenite grain size and increase toughness without large carbon increases. - Proprietary products (e.g., HARDOX) typically control trace elements and inclusion quality more tightly, giving more predictable toughness and fatigue performance.

3. Microstructure and Heat Treatment Response

Microstructure under standard processing: - Both grades are produced via quench-and-temper routes or via thermo-mechanical processing followed by controlled cooling and tempering. The goal is a tempered martensitic or bainitic-martensitic microstructure delivering high hardness with usable toughness. - HARDOX550: Manufactured to a proprietary heat-treatment cycle that produces a very uniform tempered martensite with controlled retained austenite and low inclusion content. The process is optimized for consistent through-thickness properties. - NM550: Can be produced by several mills with variable process control. Typical microstructure aimed for is tempered martensite/bainite; however, uniformity (through-thickness and plate-to-plate) may be less tightly guaranteed across suppliers.

Effect of further heat treatments: - Normalizing: Can refine grain size and homogenize the microstructure but is rarely used as the final step for 550-class wear plates; more common in plate pre-treatments. - Quenching and tempering: The principal industrial route for both; quenching establishes hard martensite, tempering reduces brittleness and tunes toughness. - Thermo-mechanical controlled processing (TMCP): Used to produce finer microstructures with lower carbon equivalents and improved toughness for a given hardness. Proprietary steels often use TMCP to enhance strength–toughness balance.

4. Mechanical Properties

Rather than presenting absolute numeric claims (which depend on supplier, thickness, and processing), the following comparative table provides typical, industry-relevant descriptors for engineers selecting between the two grades.

Which is stronger, tougher, or more ductile, and why: - Both aim for similar nominal hardness (the "550" class). Strength and hardness are comparable in the nominal sense, but HARDOX550 is typically supplied with tighter control of toughness and through-thickness properties, so its combination of strength and impact resistance is generally more predictable for critical applications. NM550 may provide similar hardness but can show greater variability in toughness and ductility, depending on supplier quality and plate thickness.

5. Weldability

Weldability considerations revolve around carbon content, alloying for hardenability, and microalloying presence. Practical guidance: - Carbon equivalents provide a first-order indication of preheating and post-weld heat treatment (PWHT) needs. Common formulas used are: - $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - Interpretation (qualitative): - Higher calculated $CE_{IIW}$ or $P_{cm}$ indicates increased risk of hard, brittle heat-affected zones and a need for controlled preheat, interpass temperature, and possibly PWHT or matching consumables. - HARDOX550 manufacturers generally provide welding guidelines, recommended consumables, and qualified procedures; they also publish limiting thicknesses and preheat recommendations. Because HARDOX550 may have higher hardenability from alloy choices, careful welding procedure control is important. - NM550 weldability depends on its carbon equivalent; some NM550 plates can be welded with standard practices and minimal preheat if CE is moderate, while others require preheat and controlled cooling to avoid cracking. - Practical tips: - Always consult the mill welding recommendations and perform procedure qualification when in doubt. - Use low-hydrogen electrodes/fillers and control interpass temperature. - Consider embossing or machining to remove hard surface layers if thermal cutting introduces excessive hardness.

6. Corrosion and Surface Protection

• Both NM550 and HARDOX550 are non-stainless wear steels; intrinsic corrosion resistance is limited and should not be relied upon for atmospheric or chemical resistance.
• Typical corrosion mitigation:
• Protective coatings: painting systems, epoxy primers, polyurethane topcoats for atmospheric environments.
• Galvanizing: possible for certain fabricated components but may require special process controls due to high hardness and potential distortion.
• Corrosion allowances: using sacrificial layers or design allowances where coating wear is expected.
• PREN (pitting resistance equivalent number) is relevant only for stainless materials with appreciable Mo and N content and is not applicable to non-stainless wear steels. For reference:
• $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• In short, for both grades, plan for surface protection in corrosive services; they are not substitutes for stainless steels.

7. Fabrication, Machinability, and Formability

• Cutting: Plasma, oxy-fuel, and laser cutting are commonly used. High-hardness weld steels can form hard recast layers or heat-affected zones—post-cut machining or grinding may be required.
• Bending/forming: At 550-class hardness, cold forming is limited; bending large deformations is generally not recommended without specialized tooling or local pre-heating. Forming is more straightforward on softer starting plates or by using incremental forming techniques.
• Machinability: Both grades are harder to machine than mild steels. Use carbide tooling, reduced feed rates, and controlled cutting parameters. Machining allowances are often specified to remove hardened surfaces.
• Finish: Grinding and shot blasting are common. Wear plates may require leveling or planing for tight flatness tolerances—this is influenced by the hardness and thickness uniformity.

8. Typical Applications

Selection rationale: - Choose plates based on abrasive load, impact severity, component geometry, needed lifetime, and repairability. If wear is dominant and service is not impact-severe, an NM550 solution from a reputable supplier may be cost-effective. If service includes impact, edge loading, or critical safety/operational considerations, a HARDOX550 solution with proven toughness and supplier support is typically preferable.

9. Cost and Availability

• Relative cost: Proprietary branded steels (HARDOX550) often carry a premium due to R&D, certification, and global supply chains. NM550, being a regional non-proprietary grade, can be less expensive on an initial purchase price.
• Availability by form: Both are available in plate form; HARDOX550 is widely available in Europe, Americas, and other markets through SSAB distribution, often with full technical support. NM550 availability is strong in regions where local mills produce NM-series wear steels; lead times and consistent thickness/hardness tolerances can vary by supplier.

10. Summary and Recommendation

Summary table (qualitative)

Conclusion and selection guidance: - Choose NM550 if: - You require a 550-class hardness wear plate and are working within a cost-sensitive budget with access to reputable regional mills. - The application is principally abrasive wear with limited high-energy impact or critical safety performance; and you can accept some variability and perform supplier qualification. - You have welding/fabrication capabilities to manage carbon equivalent and preheat requirements where necessary.

• Choose HARDOX550 if:
• You require guaranteed, documented mechanical properties (through-thickness toughness, hardness, and fatigue behavior) and supplier-backed technical support.
• The component will be exposed to severe impact, edge-loading, or critical service where predictable performance and traceability matter.
• You prefer the assurance of proprietary processing quality—even at a higher purchase price—because downtime or failure consequences are costly.

Final note: For any critical application, request mill certificates, toughness test results relevant to the expected service temperature and thickness, and welding guidelines. Perform field trials where feasible and qualify suppliers and procedures to align the selected grade’s delivered performance with the design requirements.