NM500 vs HARDOX500 – Composition, Heat Treatment, Properties, and Applications

Introduction

NM500 and HARDOX500 are high-hardness abrasion-resistant steel grades frequently considered for heavy wear components such as buckets, truck bodies, crushers, and liners. Engineers and procurement teams routinely balance competing priorities—upfront material cost, proven in-service lifetime, weldability and post-weld performance, and supplier certification—when choosing between these two families.

The practical difference most often driving a selection is how each material’s metallurgy and mill processing translate into in-service performance and expected service life. HARDOX500 is a proprietary, tightly controlled product from a premium supplier with extensive certification and predictable mechanical and wear behavior; NM500 is a widely produced abrasion-resistant grade meeting regional standards and typically offered at a lower price point. These distinctions influence design margins, joining procedures, and lifecycle cost analysis.

1. Standards and Designations

• HARDOX500: Proprietary brand name from SSAB (trade designation Hardox 500). Produced under SSAB’s own quality system and offered with manufacturer test certificates. Considered a quenched-and-tempered wear-resistant structural steel in the high-hardness category.
• NM500: Generic abrasion-resistant grade produced by multiple manufacturers; commonly referenced under Chinese standards (GB/T series) and supplier-specific specifications. It is classified as an abrasion-resistant (AR) steel.

Classification: - Both HARDOX500 and NM500 are non-stainless, low-alloy quenched-and-tempered steels in the HSLA / wear-resistant steel category rather than tool steels or stainless steels.

Applicable standards commonly referenced for wear plate steels (depending on region and supplier) include: - EN (European): EN 10051 (wear-resistant steels — general remark), EN 10025 for structural steels where applicable. - ASTM/ASME: No single ASTM grade maps directly to these proprietary/regionally-specific AR grades; ASTM A6/A36/A256 may be referenced for base steel types or testing. - GB/JIS: Local standards may specify NM series (e.g., NM500 under Chinese supplier specs). - Manufacturer specifications: SSAB product datasheets for Hardox.

2. Chemical Composition and Alloying Strategy

Representative typical composition ranges (wt%). Actual compositions vary by mill, heat, and product thickness; consult mill certificates for project-specific values.

Element	NM500 (typical range, wt%)	HARDOX500 (typical range, wt%)
C	0.10–0.25	0.18–0.25
Mn	0.60–1.60	0.80–1.60
Si	0.10–0.80	0.20–0.90
P	≤0.03	≤0.02
S	≤0.03	≤0.01
Cr	0.05–0.60	0.30–1.00
Ni	≤0.50	≤0.70
Mo	≤0.30	≤0.30
V	≤0.10	≤0.10
Nb, Ti, B	trace microalloying possible	trace microalloying possible
N	trace	trace

How alloying affects behavior: - Carbon and manganese increase hardenability and strength but raise the risk of brittle behavior and reduce weldability if not managed. - Chromium, molybdenum, and small additions of boron enhance hardenability and wear resistance by promoting fine martensitic microstructures after quenching. - Microalloying elements (V, Nb, Ti) refine grain size, improving toughness and strength balance. - Silicon supports deoxidation and can marginally strengthen; phosphorus/sulfur are controlled to avoid embrittlement and weld defects.

3. Microstructure and Heat Treatment Response

• HARDOX500: Produced using controlled quenching and tempering and, in many cases, thermo-mechanical rolling. The intended microstructure is a hardened and tempered martensitic matrix with a fine prior austenite grain size. Tight process control and consistent cooling produce uniform hardness through specified thickness ranges and good low-temperature toughness.
• NM500: Generally manufactured by quenching and tempering or heat-treated rolling; microstructure is also martensitic or heavily tempered martensite depending on heat treatment. Because NM500 is produced by many mills, microstructural uniformity and tempering response can vary more than with a proprietary product.

Effect of processing: - Normalizing before quenching can refine grain size and improve toughness. - Quenching and tempering set the hardness and balance toughness: higher tempering reduces hardness but increases toughness and ductility. - Thermo-mechanical controlled processing (TMCP) used by premium suppliers improves toughness at given hardness by producing a refined microstructure and controlled inclusion characteristics.

4. Mechanical Properties

Representative mechanical properties should be verified per mill test certificate; values below are typical ranges and depend on thickness and supplier heat treatment.

Property	NM500 (typical)	HARDOX500 (typical)
Hardness (HBW)	~470–540	~470–530 (nominal 500 HBW)
Tensile strength (MPa)	~900–1400 (varies with thickness)	~1000–1600 (varies with thickness)
Yield strength (MPa)	~700–1100	~800–1200
Elongation (A%, on gauge)	~8–20% depending on thickness	~8–18% depending on thickness
Impact toughness (J, Charpy V)	Varies; lower at higher hardness	Generally specified with guaranteed toughness values at set temperatures

Interpretation: - Both grades are engineered to provide high hardness for abrasion resistance. HARDOX500, as a controlled proprietary product, typically offers more consistent tensile–toughness balance across thicknesses and certified low-temperature impact performance. - NM500 can achieve similar hardness and tensile properties but may show wider variability in elongation and impact toughness between suppliers and heats. - For applications requiring predictable fracture toughness and guaranteed certificates, HARDOX500 often provides tighter guarantees.

5. Weldability

Weldability depends on carbon equivalent and microalloying. Two commonly used metrics are:

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

• The International Institute of Welding Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn + Cu}{20} + \frac{Cr + Mo + V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both NM500 and HARDOX500 have non-negligible carbon and alloying to obtain high hardenability; therefore, preheat and controlled interpass temperatures are commonly required to avoid cold cracking. Higher CE or Pcm values correlate with increased risk of hydrogen-assisted cold cracking. - HARDOX500 is supplied with detailed welding guidelines from the manufacturer, including recommended consumables, preheat, interpass temperature, and PWHT (if necessary), which reduces risk in critical applications. - NM500 suppliers may provide welding recommendations, but due to greater compositional variability, conservative welding parameters (higher preheat, lower interpass cooling) are often adopted in practice. - Use of low-hydrogen electrodes/fillers, appropriate matching or slightly lower-strength filler metal, and strict control of weld hydrogen content are standard practice for both grades.

6. Corrosion and Surface Protection

• Both NM500 and HARDOX500 are non-stainless carbon/alloy steels; they are not corrosion resistant by themselves. Typical protection strategies include painting, metallizing, or galvanizing depending on service conditions. For abrasive applications where coating wear is expected, sacrificial liners or design-for-replacement approaches are common.
• PREN (pitting resistance equivalent number) is not applicable for these non-stainless steels; for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index applies to stainless and duplex stainless steels, not AR carbon/alloy steels.
• In acidic or highly corrosive environments, select corrosion-resistant alloys or apply robust surface cladding (e.g., hardfacing or stainless overlays) rather than relying on base AR steel.

7. Fabrication, Machinability, and Formability

• Machinability: High hardness reduces cutting tool life and requires carbide tooling and lower cutting speeds. HARDOX500 and NM500 are both challenging to machine in the as-supplied hardened condition; recommended practice is to machine before quenching & tempering when possible, or to use appropriate tooling and coolant.
• Bending/forming: Cold forming of plate at these hardness levels is limited. Bending may lead to cracking in the tensile face; formability depends heavily on thickness and exact temper. Some forming operations are performed before final heat treatment or by using hot forming methods where feasible.
• Cutting and thermal cutting: Plasma or oxy-fuel cutting are common for plate. Thermal cutting can alter the heat-affected zone; for critical wear components, consider machining to final dimensions or removing re-heat-affected edges.
• Grinding/finishing: Surface preparation and finishing require attention to avoid introducing cracks; use qualified processes and abrasive tools.

8. Typical Applications

NM500 – Typical Uses	HARDOX500 – Typical Uses
Excavator and loader bucket liners (local/regional supply)	High-performance loader buckets, dump bodies, and wear-critical components with certified life prediction
Mining wear plates and liners (cost-sensitive projects)	Crusher liners, scale housings, and vibrating screen plates where certified toughness is required
Agricultural and earthmoving wear parts for domestic markets	Heavy-duty truck bodies and containers requiring predictable lifetime and global support
Wear plates in conveyors, chutes with design-for-replacement	Applications requiring detailed mill traceability and consistent through-thickness properties

Selection rationale: - Use NM500 where initial material cost and local availability dominate decision-making and where engineering controls are in place to manage variability. - Use HARDOX500 where predictable lifetime, strict certification, controlled toughness (especially at low temperature), and support from a single supplier network justify a premium.

9. Cost and Availability

• HARDOX500: Positioned as a premium, globally distributed product with comprehensive datasheets, supply chain traceability, and post-sale support. Unit cost is typically higher than generic alternatives.
• NM500: Often less expensive and widely available in regions with local mills producing NM-series steels. Availability in specific thicknesses and plate sizes may be greater in certain markets.
• Product form: Both grades are available as plates, cut-to-size blanks, and, for HARDOX, often as pre-cut and certified components. Lead times and availability depend strongly on regional production capacity and distributor networks.

10. Summary and Recommendation

Attribute	NM500	HARDOX500
Weldability (practical)	Good with conservative preheat; variability between suppliers requires caution	Good with manufacturer-specified welding procedures and predictable outcomes
Strength–Toughness balance	Can achieve high hardness; toughness may vary between suppliers	Engineered and certified for consistent strength–toughness and low-temperature performance
Cost	Generally lower upfront cost	Higher upfront cost; premium, traceable supply

Choose NM500 if: - Budget sensitivity and local availability are primary constraints. - The design allows for higher safety factors or frequent replacement/maintenance cycles. - The project team can control welding and fabrication tightly and accept potential variability between heats or suppliers.

Choose HARDOX500 if: - Predictable service life, tight material certification, and consistent low-temperature toughness are important. - Lifecycle cost, downtime risk, or warranty obligations favor a premium, traceable material. - The application demands consistent through-thickness properties and documented performance (e.g., critical mining or heavy transport equipment).

Final note: Always request mill/test certificates and supplier welding/fabrication instructions for the exact heat and thickness to be used. When service life is a primary metric, perform field trials or full-scale wear testing when switching between supplier grades, and incorporate manufacturer guidance into design and maintenance plans.