NM450 vs HARDOX450 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
NM450 and HARDOX450 are two frequently compared abrasion-resistant (AR) steels used in heavy-duty wear applications such as mining, earthmoving, and aggregate handling. Engineers and procurement professionals deciding between them balance competing priorities: cost and local availability versus guaranteed supplier quality, weldability and ease of fabrication versus in-service wear life, and hardness versus impact toughness.
The primary distinction between these grades lies in their alloying approach and metallurgical processing: one originates from a family of wear steels produced to meeting regional standards and economic targets, while the other is a branded, tightly controlled product with specific chemistry and processing to deliver reproducible properties. This difference manifests in composition control, microalloying strategy, and declared performance in datasheets, which is why these steels are commonly compared for design and procurement decisions.
1. Standards and Designations
- HARDOX450: Trademarked by SSAB; commonly referenced to supplier datasheets rather than a single international standard. Material is classified as quenched and tempered abrasion-resistant structural steel (HSLA-like behavior through controlled processing).
- NM450: Typically a national/regional designation for 450 HB class wear plate; may be produced to local standards such as GB/T (China) or other national specifications. Not a single global trademarked product.
Classification: - Both are non-stainless, low- to medium-alloy, high-hardness wear-resistant steels. They are not tool steels or stainless steels; they sit in the high-strength low-alloy (HSLA) / quenched-and-tempered wear steel category.
2. Chemical Composition and Alloying Strategy
Table: qualitative presence of common alloying elements (qualitative descriptors rather than precise numbers; exact values depend on supplier/specification).
| Element | NM450 (typical presence) | HARDOX450 (typical presence) | Notes |
|---|---|---|---|
| C | Medium (controlled) | Low–Medium (tightly controlled) | Carbon provides base hardenability and strength; level is controlled to balance hardness and weldability. |
| Mn | Medium | Medium | Mn aids hardenability and strength; common in both. |
| Si | Low–Medium | Low–Medium | Deoxidation and strength contribution. |
| P | Very low (controlled) | Very low (controlled) | Impurity—kept low to avoid embrittlement. |
| S | Very low (controlled) | Very low (controlled) | Impurity—kept low for toughness and weldability. |
| Cr | Trace–Low | Low (intentional micro-alloying) | Cr contributes to hardenability and wear resistance. |
| Ni | Trace | Trace | Improves toughness when present in higher amounts. |
| Mo | Trace | Trace–Low | Mo increases hardenability and high-temperature strength. |
| V | Trace–Low (possible microalloying) | Trace–Low (controlled) | Microalloying (V, Nb, Ti) helps refine grain size and improve toughness. |
| Nb (Nb/Ti) | Trace (possible) | Trace (used for grain control) | Microalloying used selectively by manufacturers. |
| Ti | Trace | Trace | Often used as a stabilizer and grain refiner. |
| B | Trace (occasionally) | Trace (occasionally) | Small boron additions can boost hardenability if used and controlled. |
| N | Residual/controlled | Residual/controlled | Nitrogen control important for toughness and fatigue performance. |
How alloying affects performance: - Carbon and manganese primarily set hardenability and as-rolled/quenched strength. - Microalloying elements (V, Nb, Ti) refine austenite grain size and improve toughness and weldability at a given hardness. - Chromium and molybdenum raise hardenability and wear resistance, enabling lower carbon contents for similar hardness targets, which improves weldability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both grades are supplied as quenched and tempered plates with a tempered martensitic/bainitic microstructure engineered to deliver high hardness while retaining some toughness. - HARDOX450 (branded) is produced under tightly controlled heat-treatment cycles and rolling schedules to obtain a homogeneous tempered martensitic structure with refined prior-austenite grain size. - NM450 (regional 450 HB class) may be produced using similar quench-and-temper or thermo-mechanical controlled processing, but supplier control and process windows can vary more between mills.
Effect of processing: - Normalizing: refines grain size and can homogenize microstructure, but alone will not achieve target hardness levels for AR plate. - Quenching & tempering: primary route to obtain the high hardness (≈450 HB) by forming martensite and then tempering to balance hardness and toughness. - Thermo-mechanical controlled processing (TMCP): used by branded producers to develop strength and toughness with precise rolling and cooling—this can reduce the need for extreme alloying and keep carbon lower.
4. Mechanical Properties
Table: qualitative comparison (both are engineered for ~450 HB class hardness; exact numeric values are supplier-dependent).
| Property | NM450 | HARDOX450 | Notes |
|---|---|---|---|
| Tensile Strength | High | High (consistent) | Both deliver high tensile strength consistent with their hardness class. Branded products often declare tighter ranges. |
| Yield Strength | High | High (consistent) | Yield can vary with supplier heat-treatment; HARDOX typically has well-characterized yield data. |
| Elongation | Moderate–Low | Moderate–Low | High hardness reduces ductility; tempered microstructures optimize residual ductility. |
| Impact Toughness | Variable (depends on mill and thickness) | Generally high for class (consistent across thickness) | HARDOX datasheets provide toughness vs thickness; NM450 toughness is more dependent on manufacturer practice. |
| Hardness | ≈450 HB class (target) | ≈450 HB class (target) | Hardness is the defining class; both aim for ~450 HB, but distribution and tolerances differ by supplier. |
Which is stronger, tougher, more ductile: - Both are engineered to the same hardness band, so nominal strength levels are comparable. The branded HARDOX450 typically provides more reproducible toughness and declared mechanical property curves across thicknesses because of stringent process control and documented testing. NM450 can provide equivalent performance, but variability between producers may be greater.
5. Weldability
Weldability depends on carbon content, overall hardenability, and microalloying.
Useful indices:
- Carbon equivalent (IIW):
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
- Pcm (for predicting cold cracking sensitivity):
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate easier weldability and lower risk of hydrogen-assisted cold cracking. Branded HARDOX450 often aims for lower carbon equivalents through controlled chemistry and TMCP to improve weldability at target hardness. - Both grades require welding procedures tailored for AR steel: preheat and interpass temperature controls, low-hydrogen consumables, and post-weld heat treatment (PWHT) considerations where necessary. - Edge preparation, local hardness peaks, and heat input must be managed to avoid brittle martensite in the heat-affected zone (HAZ). For critical fabrications, follow supplier welding guidelines.
6. Corrosion and Surface Protection
- Both NM450 and HARDOX450 are non-stainless carbon/alloy steels and do not provide inherent corrosion resistance beyond typical mild steels.
- Surface protection strategies: painting, powder coating, sacrificial or metallurgical galvanizing (where feasible), and application of wear-resistant overlays or liners.
- When corrosion resistance is a design parameter (e.g., wet, chloride-bearing environments), stainless or corrosion-resistant alloys are necessary; PREN is not applicable to these AR steels. For reference, PREN formula for stainless alloys is:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - In practice, choose coatings or specify duplex solutions (corrosion-resistant cladding or overlay) when both wear and corrosion are concerns.
7. Fabrication, Machinability, and Formability
- Cutting: Both are cut by oxy-fuel (thicker plates), plasma, laser, and waterjet; tool wear is higher due to hardness—tool selection and cutting parameters must be appropriate.
- Bending/forming: Cold forming of AR plates at ~450 HB is limited; pre-heating or hot forming (and post form heat treatment) may be required. Smaller-radius bends risk cracking or loss of hardness at the bend.
- Machinability: High hardness increases tool wear; machining is typically done in as-cut or with special tooling. Branded plates with more uniform microstructure may machine more predictably.
- Grinding/finishing: Necessary for tight tolerance edges; abrasive grinding produces high tool wear and heat—control to avoid tempering or microstructural changes.
8. Typical Applications
| NM450 (typical uses) | HARDOX450 (typical uses) |
|---|---|
| Local/regional quarry chutes, hopper liners, small conveyors (cost-sensitive projects) | High-wear truck bodies, loader buckets, excavator wear parts, mining liners where supplier support and documented performance are required |
| Screening decks, fixed liners where replacement frequency is moderate | Mobile equipment components needing reliable toughness across thickness, severe service conveyors |
| Wear plates and retrofit liners by local fabricators | Applications demanding certified material traceability and predictable in-service performance |
Selection rationale: - Choose the grade based on expected wear mechanism (sliding vs impact), required life cycle, acceptable downtime and maintenance practices, and supplier support. For high-impact plus abrasion, proven toughness across thickness and supplier QA are critical.
9. Cost and Availability
- NM450: Often more cost-competitive in markets with multiple local mills. Availability can be good regionally, but material documentation and consistent property control may vary.
- HARDOX450: Typically priced higher due to branding, guaranteed properties, global distribution, and documented testing and traceability. Availability is generally broad where the supplier has distribution networks or licensed mills.
Product forms: - Both are available as plates; HARDOX may also be available with additional services (e.g., cut-to-size, pre-qualified weld info, certified test reports) that add value but cost more.
10. Summary and Recommendation
Table summarizing qualitative comparisons:
| Criterion | NM450 | HARDOX450 |
|---|---|---|
| Weldability | Good to acceptable (varies by mill) | Good (consistent; supplier guidance available) |
| Strength–Toughness balance | High strength; toughness varies | High strength with well-characterized toughness |
| Cost | More economical (often) | Higher cost; premium for consistency and support |
Recommendations: - Choose NM450 if you need a cost-effective abrasion-resistant plate for applications with lower requirements on supplier traceability, or where local mills supply material and you can validate performance through incoming inspection and qualification testing. - Choose HARDOX450 if you require documented, reproducible mechanical properties and toughness across thickness, supplier-backed weld procedures and technical support, and minimized risk in critical, high-impact or high-consequence applications.
Final note: Regardless of grade selection, always obtain supplier datasheets, request mill test certificates, and qualify welding and fabrication procedures for the selected product and thickness. Where possible, perform application-specific wear tests or trials to confirm expected service life.