XAR400 vs NM400 – Composition, Heat Treatment, Properties, and Applications

Introduction

Selecting the right wear-resistant steel grade is a common procurement and design dilemma for engineers, manufacturing planners, and materials buyers. Decisions typically balance hardness and abrasion resistance versus weldability, toughness, and cost; production context (thickness, form, and downstream processing) heavily influences the right choice.

XAR400 and NM400 are both high-hardness, abrasion-resistant steels marketed for heavy-wear applications, but they originate from different standardization and commercial traditions and are therefore specified and produced with different alloying and processing emphases. Because designers often need to substitute or compare these grades across supply chains, understanding their chemical strategies, heat-treatment response, mechanical behavior, and fabrication implications is essential.

1. Standards and Designations

• Common international and national standards relevant for abrasion-resistant steels:
• EN (European Norms) — for structural and wear plates (e.g., EN 10029, EN 10163, and industry specifications)
• ASTM/ASME — used for mechanical testing, plates, and general references (e.g., ASTM A514/A514, A572 family have different intent)
• JIS — Japanese industrial standards for steels and plates
• GB/T — Chinese national standards for wear-resistant steels (e.g., NM series under GB/T 4171 family or related specifications)

• Proprietary commercial designations from mill producers (e.g., XAR, WELDOX, AR, Hardox)

• Material classification:

• Both XAR400 and NM400 are high-strength, wear-resistant carbon/alloy steels (commonly regarded as abrasion-resistant plates, not stainless or tool steels). They are part of the HSLA/AR family where the emphasis is on controlled chemistry and heat treatment to achieve a hard martensitic/bainitic surface combined with acceptable toughness.

2. Chemical Composition and Alloying Strategy

Below is a table of typical alloying-element ranges and common trace elements for XAR400 and NM400. These ranges reflect typical mill practice for modern abrasion-resistant plates; compositions can vary by producer and product specification. Always verify with the mill certificate for the supplied plate.

Element	Typical XAR400 (typical range)	Typical NM400 (typical range)
C (wt%)	0.08 – 0.20	0.08 – 0.20
Mn (wt%)	0.5 – 1.6	0.6 – 1.6
Si (wt%)	0.2 – 0.8	0.2 – 0.8
P (wt%, max)	≤ 0.03	≤ 0.03
S (wt%, max)	≤ 0.01 – 0.035	≤ 0.01 – 0.035
Cr (wt%)	0.2 – 1.2 (often low)	0.2 – 1.0 (often low)
Ni (wt%)	up to ~0.6 (often trace)	up to ~0.6 (often trace)
Mo (wt%)	up to ~0.3 (trace in some variants)	up to ~0.3 (trace in some variants)
V (wt%)	trace – 0.10 (if microalloyed)	trace – 0.10 (if microalloyed)
Nb (wt%)	trace (microalloying)	trace (microalloying)
Ti (wt%)	trace (occasionally)	trace (occasionally)
B (ppm)	trace (occasionally used in microalloying)	trace (occasionally)
N (ppm)	controlled levels; relevant when N is alloyed	controlled levels; relevant when N is alloyed

Notes: - XAR400 is a commercial brand family where the mill’s process control (thermo-mechanical processing, quench/tempering) and proprietary chemistry determine final properties. Some XAR variants emphasize slightly different microalloying. - NM400 is a Chinese standardized wear grade in the NM series where chemistry is set to obtain a target hardness class while allowing variants across mills. - Alloying elements such as Cr, Mo, Ni, V, Nb, and microalloying additions improve hardenability, tempering resistance, and grain refinement. Higher Mn and Cr increase hardenability but can make welding more demanding.

How alloying affects properties: - Carbon and manganese primarily control as-quenched hardness and hardenability. Higher carbon increases hardness and wear resistance but reduces weldability and toughness. - Chromium, molybdenum, and nickel increase hardenability and tempering resistance, supporting thicker sections and higher through-thickness hardness. - Microalloying elements (V, Nb, Ti) can refine grain size, improve toughness, and allow strength via precipitation hardening without excess carbon. - Sulfur and phosphorus are kept low to preserve toughness and weldability.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Both grades are produced to achieve a predominantly martensitic or martensitic–bainitic microstructure in the as-delivered plate surface to provide abrasion resistance. The core microstructure and through-thickness condition depend on thickness and the mill’s thermal route. - XAR400 (commercially processed) often uses controlled quenching, accelerated cooling, or thermomechanical rolling followed by tempering to produce a hard outer microstructure with acceptable toughness. - NM400 typically follows national production practices (controlled rolling and quenching/tempering or accelerated cooling) to reach the required hardness class with a mixture of martensite and bainite.

Response to heat treatments: - Normalizing: will refine grain size but will not reliably produce the high hardness expected of AR steels unless followed by controlled quenching. - Quenching and tempering: used to raise hardness and then temper to obtain the desired balance between hardness and toughness. Tempering temperature controls retained toughness: higher tempering reduces hardness but improves toughness and ductility. - Thermo-mechanical processing: controlled rolling and accelerated cooling in the mill can produce fine bainitic or martensitic structures with good toughness and reduced need for post-rolling heat treatment. - Post-weld heat treatment (PWHT): often avoided for AR plates in the field; local PWHT may be specified to reduce HAZ hardness and hydrogen cracking risk, but feasibility depends on component size and service.

Thickness effects: - In thicker plates, hardenability (alloy content and cooling rate) determines the depth of the hardened layer; both XAR400 and NM400 are engineered to optimize hardness at typical plate thicknesses, but mill certificates should be reviewed for guaranteed through-thickness properties.

4. Mechanical Properties

Below are typical mechanical property ranges encountered in production for these classes of wear-resistant plate. These are representative ranges and can vary by mill, tempering practice, and thickness.

Property	Typical XAR400 (typical range)	Typical NM400 (typical range)
Hardness (HBW)	~360 – 440	~360 – 440
Tensile Strength (MPa)	~900 – 1400	~900 – 1400
Yield Strength (MPa)	~600 – 1100 (dependent on definition and thickness)	~600 – 1100
Elongation (%)	8 – 20 (thickness dependent)	8 – 20 (thickness dependent)
Impact Toughness (Charpy V, J)	Variable: low to moderate at low temperatures; mill-specific (e.g., 10–40 J indicative for thinner sections)	Variable: low to moderate; mill-specific

Interpretation: - Hardness is the primary controlled attribute for both grades; the numbers above are indicative of a “400 HB” class. - Tensile and yield strengths scale with hardness and tempering; higher hardness correlates with higher strength but lower ductility. - Toughness (impact energy) is sensitive to composition, rolling and cooling schedules, and thickness; some commercial XAR variants may offer improved through-thickness toughness due to proprietary processing. - Neither grade is inherently “stainless.” Corrosion resistance is limited and requires surface protection in corrosive environments.

5. Weldability

Weldability assessment focuses on carbon content, alloying, and hardenability. Two commonly used empirical indices are the IIW carbon equivalent and the Pcm formula.

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

• Dearden–Baxter / 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: - Higher $CE_{IIW}$ or $P_{cm}$ values indicate greater risk of HAZ hardening and cold cracking; preheat, controlled interpass temperature, and low-hydrogen consumables are required as indices rise. - Both XAR400 and NM400 typically have modest carbon with alloying tuned to provide hardenability; however, the combined effect can produce significant HAZ hardness if standard mild-steel welding practices are used. Therefore: - Preheat and controlled welding procedures are commonly required for thicknesses above certain limits. - Use of low-hydrogen electrodes/flux-cored arcs and matched or lower strength filler materials to avoid over-hardening the HAZ is recommended. - When joining wear plates to mild steel, designing transition joints or thermally conditioning welds reduces cracking risk. - Practical advice: always review mill-provided welding guidelines and qualify procedure specifications (WPS/PQR) for the assembled thickness and service conditions.

6. Corrosion and Surface Protection

• Both grades are carbon/alloy steels (not stainless); corrosion resistance is limited. Typical protection strategies:
• Painting or solvent-borne epoxy coatings for general environments.
• Hot-dip galvanizing for atmospherically exposed components where galvanizing is feasible (consider thickness limits and adhesion for highly hard surfaces).
• Metalizing (thermal spray) or cladding where both wear and corrosion resistance are needed.

• For slurry environments, sacrificial or hard overlay weld cladding (stainless or cobalt-based) may be specified.

• PREN is not applicable to non-stainless steels; it is calculated as: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is meaningful only for corrosion-resistant stainless alloys and thus not relevant for XAR400 or NM400.

7. Fabrication, Machinability, and Formability

• Cutting: Abrasion-resistant plates cut using oxy-fuel, plasma, laser, or waterjet. Hardness and possible surface hardening demand appropriate consumables (plasma with high power, waterjet for precision).
• Machinability: High hardness reduces machinability — carbide tooling, reduced feed rates, and appropriate coolant are recommended for post-machining.
• Bending/forming: Cold forming of AR plates is limited; spring-back and risk of cracking increase with hardness. Forming is usually done either with heated forming processes or by bending on blanks with large radii; some operations opt for forming in softer pre-quenched condition followed by local hardening.
• Surface finishing: Grinding or shot-blasting may be required to prepare surfaces for welding or coating. Maintain awareness that grinding can locally remove hardfinishing layer and change wear performance.

8. Typical Applications

XAR400 — Typical Uses	NM400 — Typical Uses
Excavator bucket edges, dump truck bodies, liners for chutes and hoppers	Earthmoving bucket wear plates, crusher liners, screening equipment
Crusher and mill liners in mining and aggregates	Wear liners in cement plants, coal handling, and dredging equipment
Conveyor skirtboards and wear strips in high-impact abrasion	Wear plates in heavy equipment manufactured within local/regional supply chains
Applications where proprietary mill processing is specified for toughness in a given thickness	Applications where standardized national grade and broad supplier base are prioritized

Selection rationale: - Choose a grade (and supplier) whose hardness and toughness performance have been verified for the specific thickness and loading condition. For high-impact abrasion, consider trade-off between hardness and toughness; a slightly lower hardness with better toughness is preferable in applications with heavy impact. - Procurement often hinges on available plate sizes, local supplier familiarity, and certification (mill test reports).

9. Cost and Availability

• Cost drivers: mill processing (thermo-mechanical rolling, quenching/tempering), alloy additions, plate thickness, and logistics.
• Availability: XAR is a commercial brand available from mills with established export channels; NM400 is produced widely within China and neighboring markets under standardized NM specifications and may be more readily available and cost-effective in those regions.
• Product form: both grades are available as plates; availability in cut-to-length, pre-hole, or welded assemblies varies by supplier and market.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	XAR400	NM400
Weldability	Moderate; requires qualified procedures and preheat guidance; mill instructions vary	Moderate; similar considerations; check mill-specific guidance
Strength–Toughness balance	Engineered by mill; some XAR variants emphasize enhanced toughness for given hardness	Designed to achieve standardized hardness class; toughness varies with mill practice
Cost / Local availability	May be higher or variable depending on brand and region	Often cost-competitive and widely available in regions covered by national producers

Recommendations: - Choose XAR400 if you need a commercially branded plate with documented mill-controlled processing where verified through-thickness toughness at a specified hardness is critical and you can source the brand within acceptable cost and lead-time. - Choose NM400 if you prefer a standardized, widely available wear plate with cost advantages in local/regional markets and when supplier mill certificates and performance testing (hardness, impact) meet application requirements.

Final practical notes: - Always obtain mill test reports (chemical analysis, hardness maps, and mechanical tests) for the specific plate thickness and heat number. - Pre-qualify welding procedures and validate performance with representative coupons for the intended thickness and service. - For critical components exposed to combined corrosion and abrasion or severe impact, consider wear-resistant overlays or engineered composites rather than relying solely on base plate hardness.