NM360 vs NM400HB – Composition, Heat Treatment, Properties, and Applications

Introduction

NM360 and NM400HB are two widely used abrasion‑resistant (AR) steel grades encountered in mining, quarrying, earthmoving, and bulk‑handling equipment. Engineers, procurement managers, and manufacturing planners often weigh wear resistance, toughness, weldability, and cost when selecting between them. Typical decision contexts include whether a part must resist severe abrasive wear at the expense of some ductility, or whether repeated impact and fatigue demand a tougher, less brittle material.

The primary operational distinction between the two is their target hardness and the resulting balance of strength versus toughness: one grade is selected for somewhat lower hardness with better ductility and impact performance, while the other is specified for higher Brinell hardness and superior abrasive resistance. Because both are marketed for wear applications and are often produced by multiple mills under similar trade names, comparisons focus on chemistry strategy, microstructure produced by heat treatments, mechanical properties, weldability, and practical fabrication considerations.

1. Standards and Designations

• Common national and international contexts where similar grades appear:
• GB (China): NM series (NM360, NM400, etc.) — often used in Chinese standards and supplier specs.
• EN (Europe): EN 1.XXX designations are less commonly used for AR steels; AR grades may be listed in supplier norms rather than a single EN number.
• JIS (Japan): Wear steels are often specified by supplier trade names rather than a single JIS number.
• ASTM/ASME (USA): AR steels are commonly referenced by trade name (e.g., AR400, AR360) and by product standards for plate and hardness testing.
• Classification: Both NM360 and NM400HB are non‑stainless, low‑to‑medium alloy high‑hardness steels formulated primarily as abrasion‑resistant (AR) steels; they are not tool steels or stainless steels and are best treated as wear‑resistant carbon‑microalloyed or alloy steels (HSLA tendencies for strength control).

2. Chemical Composition and Alloying Strategy

The precise chemical composition of NM360 and NM400HB varies by supplier and country standard. Rather than quoting fixed values, the following table summarizes the typical alloying strategy and relative presence of common elements used in AR steels of these target hardness classes.

Element	Typical presence / role in NM360	Typical presence / role in NM400HB
C (carbon)	Low–moderate; provides hardenability and strength but balanced for toughness	Moderate; slightly higher to achieve higher hardenability and hardness
Mn (manganese)	Moderate; deoxidation, solid solution strengthening, improves hardenability	Moderate to higher; increases hardenability and tensile strength
Si (silicon)	Minor to moderate; deoxidizer, strengthens ferrite	Minor; similar role
P (phosphorus)	Kept low (impurity control) for toughness	Kept low
S (sulfur)	Kept low for toughness; manganese sulfides may aid machinability	Kept low
Cr (chromium)	Often present in small amounts or omitted; improves hardenability and wear property	Often present as small additions to increase hardenability and tempering resistance
Ni (nickel)	Small or none; used when improved toughness at low temperatures is required	Occasionally used in small amounts for toughness improvement
Mo (molybdenum)	Trace/low; increases hardenability and tempering resistance	Trace/low; used to increase hardenability and strength after tempering
V (vanadium)	Trace microalloying; grain refinement when present	Trace microalloying; grain refinement and precipitation strengthening
Nb (niobium)	Trace microalloying in some thermo‑mechanical processes for grain control	Trace, when specified
Ti (titanium)	Trace for deoxidation and inclusion control	Trace
B (boron)	Trace additions possible to boost hardenability at ppm levels	Trace possible in some heat‑treated products
N (nitrogen)	Low; controlled for inclusion and CE/Pcm calculations	Low

Notes: Supplier specifications or national standards provide exact ranges. Small additions of Cr, Mo, V, or B are commonly used to tune hardenability and tempering response; however these are typically at low absolute concentrations. The key compositional strategy is to balance carbon and manganese for hardenability while using microalloying and small alloy additions to preserve toughness and refine grain size.

How alloying affects properties: - Carbon and manganese primarily control hardenability and achievable hardness after quench/tempering; increasing them raises hardness and strength but can reduce ductility and weldability. - Microalloying elements (V, Nb, Ti) refine prior austenite grain size and aid strength without large sacrifices in toughness. - Small Cr and Mo additions increase tempering resistance, improving wear performance at elevated service temperatures and under impact.

3. Microstructure and Heat Treatment Response

Typical microstructures for AR steels targeted at 360 HB and 400 HB depend strongly on processing:

• NM360 (lower hardness target):
• Typical microstructure: tempered martensite and/or bainite with a relatively finer distribution of carbides; may include retained austenite in some formulations.

• Processing: often produced by controlled hot rolling followed by quench and temper, or by quench plus temper with milder quench or lower tempering temperatures to yield a balance of hardness and toughness. Thermo‑mechanical rolling can produce fine bainitic structures with improved toughness.

• NM400HB (higher hardness target):

• Typical microstructure: higher fraction of martensite and harder bainitic constituents; carbide dispersion and potential retained austenite depend on steel chemistry and cooling rate.
• Processing: requires stronger quench or lower temper to reach higher Brinell hardness; some producers use alloy additions (Cr, Mo, B) to increase hardenability so thicker sections can achieve uniform higher hardness. Quench & temper cycles are adjusted to limit embrittlement.

Effect of heat treatment routes: - Normalizing: refines grain size and is sometimes specified as a pre‑treatment but will not reach final hardness targets alone. - Quenching & tempering: primary route to achieve specified hardness levels; tempering temperature controls the hardness–toughness tradeoff. - Thermo‑mechanical rolling (controlled rolling): can produce bainitic/tempered microstructures with excellent toughness at a given hardness, improving impact resistance compared to coarse‑grained quench‑tempered steel.

4. Mechanical Properties

Exact mechanical properties depend on composition, heat treatment, and plate thickness. The table below gives comparative behavior rather than single guaranteed numbers; hardness values reflect the grade target.

Property	NM360 (typical behavior)	NM400HB (typical behavior)
Tensile strength	Moderate to high; adequate for wear parts; lower than NM400HB when both produced to hardness spec	Higher tensile strength consistent with higher hardness
Yield strength	Moderate; depends on heat treatment	Higher yield strength
Elongation (ductility)	Higher ductility/elongation compared with NM400HB	Reduced elongation due to higher hardness
Impact toughness	Better impact and fracture toughness at comparable thickness	Lower impact toughness unless chemistry and heat treatment are optimized
Hardness (Brinell)	Approximately in the 350–370 HB ballpark (grade name target)	Approximately 400 HB target (designation indicates higher HB)

Interpretation: - NM400HB will typically be stronger and offer superior abrasive resistance because of the higher hardness, but this comes at the expense of ductility and impact resistance unless mitigated by careful chemistry and thermo‑mechanical processing. - NM360 provides a more favorable balance when parts are subject to combined impact and abrasion or when deformation and forming are required before service.

5. Weldability

Weldability of AR steels depends on carbon equivalent (hardenability) and microalloying content; thicker sections and higher hardenability increase the risk of weld‑zone cracking and embrittlement. Common predictive formulas are useful for qualitative interpretation:

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

and

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

• Interpretation: higher values of $CE_{IIW}$ or $P_{cm}$ indicate greater risk of hard, brittle heat‑affected zones (HAZ) and the need for preheat, controlled interpass temperature, or post‑weld heat treatment (PWHT).
• Relative weldability: NM360 generally welds more easily than NM400HB because of its lower carbon/hardenability target; NM400HB may require more stringent welding practices, lower heat input, preheat, or softening treatments in the HAZ for thicker sections.
• Practical guidance: use low‑hydrogen welding consumables, control interpass temperature, and consider PWHT or post‑weld tempering for thicker plates or in demanding service. Prequalification weld procedures are recommended for critical components.

6. Corrosion and Surface Protection

• Both NM360 and NM400HB are non‑stainless carbon/alloy steels and have comparable baseline corrosion resistance; they are not suitable for corrosive environments without protection.
• Common protection strategies: painting, primer systems, metallizing, or hot‑dip galvanizing where appropriate. For parts subject to wear, protective coatings must be compatible with abrasion; sacrificial coatings rarely survive heavy abrasive service for long.
• PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels. For reference when stainless alloys are considered, one uses: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Clarification: corrosion protection selection is driven by environmental exposure and service life expectations; often, thicker material and planned replacement cycles are chosen for heavily abrasive, corrosive services.

7. Fabrication, Machinability, and Formability

• Cutting: Higher hardness of NM400HB increases tool wear; abrasive‑resistant plates often require carbide or ceramic tooling and slower cutting speeds compared with softer structural steels.
• Bending/forming: NM360, with its lower hardness and higher ductility, is easier to bend or cold‑form. NM400HB has reduced formability; bending may induce cracking unless using larger bend radii or warm forming techniques.
• Machinability: Both are tougher to machine than mild steel; NM400HB is generally more challenging. Consumable and tool selection should account for abrasive wear and harder microstructure.
• Finishing: Grinding and surface finishing are more intensive on NM400HB; selection of abrasives and dressing frequency must anticipate faster wear of grinding media.

8. Typical Applications

NM360 (typical uses)	NM400HB (typical uses)
Bucket liners for loaders where impact and abrasion coexist	Crusher liners and grizzly bars where severe abrasion dominates
Chute and hopper liners where moderate abrasive wear and occasional impact occur	Wear plates in high‑abrasion conveyors and pulverizers where high hardness extends life
Earthmoving equipment components requiring some formability in manufacturing	High‑wear surfaces that are preformed and welded into assemblies
Screening plates and light to medium duty liners	Applications where maximum service life against abrasion is required and cost of replacement is high

Selection rationale: - Choose NM360 where parts require a balance of abrasion resistance and toughness, or where fabrication operations (bending, forming) occur before installation. - Choose NM400HB where maximized resistance to abrasive wear is the priority and components are designed and fabricated to avoid impact or catastrophic overload.

9. Cost and Availability

• Cost: NM400HB commonly commands a premium relative to NM360 because achieving higher hardness usually requires tighter chemistry control, more processing (controlled quench/temper), and potentially higher alloy content or thermo‑mechanical processing. However, cost differentials vary by mill, region, and plate size.
• Availability: Both grades are widely available in plate form from major suppliers, with stock sizes and lead times depending on market demand and mill capability. NM360 variants are often more common in mixed‑wear applications; NM400 (HB) is produced where markets demand higher hardness AR plate.
• Product forms: Available as plate, fabricated liners, and sometimes as welded assemblies or overlayed parts; specialized sizes or tight hardness tolerances may increase lead time and cost.

10. Summary and Recommendation

Criterion	NM360	NM400HB
Weldability	Better (lower hardenability risk)	More demanding (higher HAZ cracking risk)
Strength–Toughness balance	Better toughness and ductility at working hardness	Higher hardness/strength; lower ductility unless optimized
Cost (relative)	Typically lower	Typically higher due to higher hardness processing

Choose NM360 if: - The component will experience combined impact and abrasion, where toughness and ductility are critical. - Parts require bending, cold forming, or more straightforward fabrication. - Weldability and lower preheat/PWHT demands are preferred. - Slightly lower material cost and easier machining are priorities.

Choose NM400HB if: - Abrasion resistance is the overriding requirement and higher Brinell hardness will meaningfully extend service life. - The part is designed and fabricated to avoid heavy impact or brittle fracture (e.g., replaceable wear liners, bolted or welded assemblies planned for hard service). - The project can accommodate stricter welding controls, more intensive machining/finishing, and potentially higher material expense in exchange for longer wear life.

Final note: Because actual chemical ranges, heat‑treatment schedules, and mechanical guarantees differ between suppliers, engineers should request mill certificates and qualify weld procedures and mechanical tests for the specific plate batch and thickness proposed for critical applications. Material selection should balance life‑cycle cost, fabrication practicality, and service environment rather than rely on grade name alone.