NM400 vs AR400 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
NM400 and AR400 are two widely referenced abrasion-resistant steels used for highly wear-exposed components such as buckets, liners, chutes, and crushers. The selection dilemma engineers face is often a trade-off among wear resistance (hardness), toughness (resistance to cracking under impact), weldability and fabrication cost. Typical decision contexts include choosing a plate for heavy-impact, high-abrasion service where toughness is critical (e.g., mining buckets), versus a primarily sliding-wear environment where maximum surface hardness and cost efficiency are prioritized.
The key distinguishing factor between these two labels is their origin in different specification traditions and mill practices: one is more commonly supplied under Chinese NM-series specifications and the other under American/AR-style commercial designations. Because each label is a commercial grade rather than a single fully harmonized international standard, direct equivalence requires checking mill certificates, mechanical tests and hardness targets rather than assuming identical chemistry.
1. Standards and Designations
- AR400: Commonly a commercial abrasion-resistant ("AR") designation used in North America and by many mills globally; often supplied to customer- or mill-specific standards with a nominal Brinell hardness around 400 HBW. Not a single ASTM standard but frequently produced to recognized AR plate specifications.
- NM400: Typically a designation used in Chinese standards for wear-resistant steel plate (the NM-series); commonly referenced to GB/T or supplier specs with nominal hardness near 400 HBW.
- Other related standards and notations engineers may encounter:
- ASTM/ASME: No single ASTM grade "AR400" but plates are often supplied to ASTM A36, A572, A514 etc. when special processing is applied; mileage varies by supplier.
- EN: European norms classify wear-resistant plates by hardness classes (e.g., HARDOX 400 is a proprietary grade from Sweden).
- JIS: Japan has its own wear-resistant series (e.g., SNCM, other designations).
- GB: Chinese GB/T system includes NM grades (NM400, NM450, etc.).
- Category: Both NM400 and AR400 are carbon–low-alloy, quenched and tempered high-hardness steels commonly regarded as wear-resistant structural steels (not stainless, not tool steels in the high-alloy sense). They are often treated as HSLA in the sense of controlled compositions and thermomechanical processing to achieve a balance of hardness and toughness.
2. Chemical Composition and Alloying Strategy
Note: Composition ranges below are representative, commonly reported by mill product literature. Actual composition can vary by producer and specification — always review the mill certificate (MTC).
| Element | Typical NM400 (approx.) | Typical AR400 (approx.) |
|---|---|---|
| C | 0.12 – 0.25 wt% | 0.10 – 0.20 wt% |
| Mn | 0.8 – 1.6 wt% | 0.4 – 1.2 wt% |
| Si | 0.2 – 0.8 wt% | 0.1 – 0.5 wt% |
| P | ≤ 0.035 wt% | ≤ 0.03 wt% |
| S | ≤ 0.035 wt% | ≤ 0.03 wt% |
| Cr | trace – 0.4 wt% | trace – 0.4 wt% |
| Ni | trace – 0.5 wt% | trace – 0.5 wt% |
| Mo | trace – 0.2 wt% | trace – 0.2 wt% |
| V | trace – 0.1 wt% | trace – 0.1 wt% |
| Nb (Cb) | trace – 0.05 wt% | trace – 0.05 wt% |
| Ti | trace | trace |
| B | trace (rare) | trace (rare) |
| N | low (mill-controlled) | low (mill-controlled) |
How alloying affects performance: - Carbon (C): Primary strengthening element to reach required hardness; higher C raises hardenability and wear resistance but reduces weldability and toughness. - Manganese (Mn): Improves hardenability and tensile strength; helps deoxidation; higher Mn in NM grades often boosts toughness but also increases the need for preheat on welding. - Silicon (Si): Deoxidizer and strength contributor; moderate amounts help strength without significantly hurting toughness. - Microalloying elements (Cr, Mo, V, Nb): Added in small amounts in some mill recipes to increase hardenability, tempering resistance and grain refinement, improving the strength–toughness balance. - Impurities (P, S): Kept low to preserve toughness and weldability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both NM400 and AR400 are produced by controlled rolling and quenching & tempering to generate a predominantly tempered martensitic microstructure, often with some tempered bainite depending on cooling rate and chemistry. - Microalloying and prior austenite grain size control strategies influence the final distribution of carbides and residual austenite.
Heat-treatment/processing responses: - Normalizing: Raises toughness and refines grain size but will not produce the high wear hardness required; used for stress-relief or ahead of final quench/temper cycles. - Quenching & tempering: The standard route to reach 360–440 HB. Quench creates hard martensite; tempering adjusts hardness–toughness trade-off. Higher temper temperature lowers hardness and raises toughness. - Thermo-mechanical controlled processing (TMCP): Used by some mills (notably for NM-series) to obtain fine-grained structures giving superior toughness at a given hardness compared to simple quench–tempered products. - Response differences: NM400 variants from TMCP routes may show somewhat better low-temperature toughness for the same nominal hardness compared with simpler AR400 chemistry that relies more heavily on quench/temper routes.
4. Mechanical Properties
Representative (approximate) mechanical property ranges — verify per supplier certificate.
| Property | NM400 (approx.) | AR400 (approx.) |
|---|---|---|
| Tensile strength | 900 – 1,200 MPa | 850 – 1,150 MPa |
| Yield strength (0.2% offset) | 700 – 1,000 MPa | 650 – 950 MPa |
| Elongation (A%) | 8 – 18% | 8 – 16% |
| Impact toughness (Charpy V, room temp or -20°C) | Highly variable; often moderate–good depending on processing | Variable; can be lower if chemistry/hardenability higher |
| Hardness (Brinell) | ~360 – 440 HB | ~360 – 440 HB |
Explanation: - Strength and hardness are primarily controlled by heat treatment (quench/temper) and carbon/hardenability. Both grades are supplied to similar hardness windows and therefore often have overlapping tensile/yield ranges. - Toughness differences are influenced by chemistry, inclusion cleanliness, and thermomechanical processing. NM400 products produced under strict TMCP recipes can offer a little better toughness at equivalent hardness compared to some AR400 offerings, but this is supplier-dependent. - Ductility (elongation) tends to decrease as hardness increases; lower-carbon, fine-grained variants retain more elongation.
5. Weldability
Weldability depends on carbon content, combined-alloy content (hardenability), and the presence of microalloying elements.
Useful weldability indices: - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (more conservative index for preheat): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Higher $CE_{IIW}$ or $P_{cm}$ values indicate increased risk of hydrogen-assisted cold cracking and the need for preheat, interpass temperature control and possibly PWHT (post-weld heat treatment). - NM400 variants may contain higher Mn or controlled microalloying to improve toughness and hardenability; this can raise weldability indices slightly relative to lower-alloy AR400 compositions, increasing preheat requirements. - AR400 with lower carbon and alloying is often somewhat easier to weld, but because both types are intended to be hardenable, localized HAZ hardening and cracking remain concerns. - Best practice: follow supplier weld procedures, use low-hydrogen consumables, apply appropriate preheat and interpass controls, and verify weld HAZ toughness with qualification tests for critical components.
6. Corrosion and Surface Protection
- Both NM400 and AR400 are non-stainless carbon/alloy steels. They do not provide corrosion resistance beyond ordinary carbon steel.
- Recommended protective measures: painting, powder coating, galvanizing (where appropriate), or sacrificial coatings. For parts that operate in corrosive environments combined with abrasion, consider overlay welding with corrosion-resistant alloys or specifying stainless wear liners.
- PREN (pitting resistance equivalent number) is not applicable for these non-stainless steels but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Use PREN only when evaluating stainless grades; NM400/AR400 should be specified with appropriate surface protection if corrosion exposure is expected.
7. Fabrication, Machinability, and Formability
- Cutting: Abrasion-resistant plate at ~400 HB is difficult to cut by conventional oxy-fuel methods without preheating. Plasma and laser cutting are common; waterjet cutting is ideal for heat-free cutting.
- Machinability: Low — the high hardness and work-hardening tendency make conventional machining slow and wearing on tools; carbide tooling and optimized machining strategies are required.
- Bending/Forming: Cold forming is limited and typically not recommended at full hardness. Bending may cause cracking in the HAZ or fracture on the tensile side. Where forming is required, consider hot-forming or pre-annealing and re-hardening, or use manufactured bends from suppliers.
- Finishing: Grinding and surface profiling require robust tooling and coolant; abrasive grinding is common for finishing weld seams or fitting.
8. Typical Applications
| NM400 (Examples) | AR400 (Examples) |
|---|---|
| Mining buckets and liners where impact and abrasion occur; conveyors in heavy mining operations | Excavator buckets, crusher liners, wear plates for recycling and quarrying |
| Heavy-duty crusher jaws, chutes, and hoppers with combined impact and sliding wear | Truck bodies and dump liners for abrasive aggregates |
| TMCP-produced wear parts where enhanced low-temperature toughness is needed | General-purpose wear plates where consistent hardness and availability are important |
Selection rationale: - Choose a specific product (NM400 or AR400) based on the dominant wear mechanism: sliding wear favors higher hardness; impact + abrasion requires better toughness. Also consider supplier track record, test certificates, and field performance for the particular service.
9. Cost and Availability
- Cost: Relative cost depends on market region and certification. AR400 plate from large Western mills may command a premium where traceability, certifications and plate flatness are critical. NM400 from regional mills can be cost-competitive, especially in Asia, but certified variants with TMCP or enhanced toughness treatments may cost more.
- Availability by form: Both grades are available as plates, but lead times vary by thickness, plate size and local inventory. Proprietary substitutes (e.g., HARDOX) may have premium pricing and controlled supply channels.
- Procurement note: Always request mill test certificates (chemical analysis, hardness, mechanical tests) and supplier-specific fabrication/welding recommendations. Total life-cycle cost (replacement frequency, downtime for repair, weld repairability) often outweighs first-cost decisions.
10. Summary and Recommendation
| Parameter | NM400 | AR400 |
|---|---|---|
| Weldability | Fair (depends on Mn and alloying; may need preheat) | Fair to Good (often slightly easier if lower CE) |
| Strength–Toughness balance | Good (TMCP variants can offer superior toughness at hardness) | Good (overlap in properties; supplier-dependent) |
| Cost | Competitive (region-dependent) | Competitive to premium (region- and supplier-dependent) |
Conclusion and practical guidance: - Choose NM400 if: - You need a wear plate specified under the NM-series or GB/T-style supply chain and want potential benefits from TMCP-produced material with better toughness at a given hardness. - Your service includes combined impact and abrasion where toughness is critical and the supplier can provide toughness data and welding procedures. - Cost and regional availability favor NM-series mills and you have access to required mill certifications.
- Choose AR400 if:
- You require a widely recognized AR-style product in markets where AR400 is the dominant, readily available commodity and supplier welding/fabrication practices are established.
- Your application emphasizes straightforward abrasion resistance with moderate impact and you want simpler, more common procurement across North American or global supply chains.
- You prefer a grade with potentially lower carbon/alloying within the same hardness band for slightly easier welding.
Final recommendation: Do not assume direct interchangeability on name alone. Specify the required hardness, minimum impact energy (if impact service), thickness, and require a mill certificate with chemical analysis and mechanical tests. For welded assemblies or critical components, request weld procedure specifications and, where necessary, full-scale qualification testing. Selecting the right product is a combination of hardness target, verified toughness, weldability constraints and supplier quality — not just the grade label.