NM400 vs NM360 – Composition, Heat Treatment, Properties, and Applications

Introduction

NM360 and NM400 are two widely used quenched-and-tempered wear-resistant steels produced for applications where abrasion resistance is a primary requirement. Engineers, procurement managers, and manufacturing planners commonly face a selection dilemma between these grades: trading increased hardness and wear life against weldability, formability, and cost. Typical decision contexts include choosing a grade for mining and quarrying wear parts (where maximum abrasion resistance is needed) versus selecting material for applications where fabrication and impact toughness are more important than absolute hardness.

The principal practical distinction between the two grades is that NM400 is specified to deliver higher hardness and wear resistance than NM360. That difference is achieved primarily through a modest increase in carbon and/or alloying and through tighter control of thermomechanical processing and heat treatment. The resulting trade-offs drive the common comparisons made during design and procurement.

1. Standards and Designations

• Common national and international standards where you will find wear-resistant steels and equivalents:
• GB (China): NM360, NM400 are Chinese designations commonly used in GB/T standards for wear-resistant steels.
• EN (Europe): Often compared to EN-designated abrasion-resistant steels (e.g., AR/Hardox equivalents), though direct one-to-one mapping requires checking chemistry and hardness.
• JIS (Japan): Has its own abrasion-resistant steel designations.
• ASTM/ASME (USA): No direct NM-series names; typical equivalents are designated by hardness or functional grade (e.g., AR400).
• Classification: Both NM360 and NM400 are high-strength, low-alloy (HSLA) abrasion-resistant carbon steels produced by quenching and tempering (quenched & tempered carbon/low-alloy steels), not stainless or tool steels.

2. Chemical Composition and Alloying Strategy

The following table summarizes the usual focus elements for NM-type abrasion-resistant steels. Instead of single-source numeric values (which vary by producer and standard), the table uses qualitative presence or relative level annotations that reflect typical supplier practice.

How alloying affects performance - Hardenability & strength: Elements such as C, Mn, Cr, Mo and small additions of B/Nb/V increase the steel’s ability to form martensite/bainite on quenching, thus increasing hardness and tensile strength. - Toughness: Low impurity levels (P, S) and careful microalloy additions (Nb, V, Ti) plus optimized heat treatment preserve impact toughness. - Corrosion: These are non-stainless steels; alloying here is not focused on corrosion resistance.

3. Microstructure and Heat Treatment Response

Typical starting microstructures for NM-type steels after hot rolling are ferrite–pearlite or bainitic-ferritic depending on composition and cooling rate. Final properties are produced by controlled quench-and-temper or thermomechanical controlled processing (TMCP).

• NM360: Designed to achieve a balance between hardness and toughness. After quench and temper, microstructure typically contains tempered martensite and/or lower bainite with fine carbide dispersions. The lower carbon and alloy content vs NM400 produce slightly higher retained ductility and generally easier tempering response.
• NM400: Targets a harder, more wear-resistant microstructure—typically tempered martensite with higher dislocation density and finer carbide precipitation. Higher carbon and controlled alloying or microalloying increase hardenability, enabling higher hardness for a given thickness after quenching and tempering or TMCP.

Heat treatment effects - Normalizing: Refines grain and slightly increases strength and hardness but is insufficient alone to reach target wear hardness—usually followed by quench & temper for both grades. - Quenching & Tempering: Produces the desired combination of hardness and toughness. Increased tempering temperature lowers hardness but improves toughness. - Thermo-mechanical treatment (TMCP): Can produce fine-grained bainitic/martensitic microstructures with excellent toughness at high hardness, particularly important for thicker plates to avoid excessive hard zones.

4. Mechanical Properties

The mechanical properties of NM360 and NM400 are differentiated primarily by hardness target. Hardness is often the specified performance metric because wear life correlates strongly with hardness in many abrasive conditions.

Interpretation - NM400 is intentionally stronger and harder than NM360, giving improved abrasive wear resistance at the expense of some ductility and potentially lower impact toughness if not processed carefully. - The magnitude of the trade-off depends on thickness, heat treatment, and exact chemical composition; modern TMCP and microalloying can reduce these trade-offs.

5. Weldability

Weldability is governed principally by carbon content, alloying, and hardenability. Higher carbon and alloy contents increase the risk of cold cracking and require more stringent preheat and interpass controls.

Useful carbon-equivalent metrics: - IIW carbon equivalent (qualitative weldability indicator): $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$ - International Institute of Welding Pcm formula (for preheat guidance): $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$

Qualitative interpretation - NM400, with its higher nominal carbon/alloying and hardness target, will typically have a higher $CE_{IIW}$ and $P_{cm}$ than NM360, indicating more restrictive welding procedures (higher preheat, slower interpass cooling, use of low-hydrogen consumables). - Mitigation measures: controlled preheat and interpass temperatures, matched or over-matching filler metals with adequate toughness, post-weld heat treatment (PWHT) where required, and strict control of moisture to limit diffusible hydrogen. - For heavy fabrication, welding procedures must be qualified for plate thickness and grade to avoid HAZ hardening and cold-crack susceptibility.

6. Corrosion and Surface Protection

• Non-stainless: Both NM360 and NM400 are non-stainless carbon/low-alloy steels. They are not designed for corrosion resistance.
• Surface protection options: galvanizing (hot-dip or pre-coated), protective paints, powder coatings, and sacrificial coatings. Note that galvanizing or intense thermal coatings can affect surface hardness or introduce localized stresses; consider coating process compatibility with final hardness and wear requirements.
• PREN: The pitting resistance equivalent number, $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ is not applicable for these non-stainless grades; corrosion protection should be treated by coating selection and cathodic protection strategies as appropriate.

7. Fabrication, Machinability, and Formability

• Machinability: NM400 is harder to machine than NM360 due to higher hardness and strength; tool wear is increased. Use carbide tooling, reduced feed rates, and optimized cutting parameters. Pre-machining before final heat treatment can be advantageous.
• Formability: Cold forming is limited for both grades compared with low-carbon mild steels; NM360 offers better bendability than NM400. Where complex forming is required, form in softer condition before final quench & temper or use thermal/mechanical paths that improve formability.
• Joining & assembly: Mechanical fastening is common in applications where welding could impair local wear performance; consider bolted assemblies with hard-faced wear parts for replaceability.

8. Typical Applications

Selection rationale - Choose NM360 when moderate abrasion resistance, better ductility/formability, and easier welding are priorities or when parts are thinner and impact loads moderate. - Choose NM400 where life extension under severe abrasive wear justifies higher material and processing costs and where fabrication methods can accommodate stricter welding/forming controls.

9. Cost and Availability

• Cost: NM400 is generally more expensive per kilogram than NM360 because of higher alloying/hardness targets and more rigorous processing. Real cost depends on supplier, plate thickness, and heat-treatment consistency.
• Availability: Both grades are commonly available in plate form from major steelmakers; however, very high hardness plate in large thicknesses may have longer lead times or limited stock. NM360 may be more readily stocked in a wider range of thicknesses and dimensions.

10. Summary and Recommendation

Summary table

Recommendation - Choose NM400 if you require maximum abrasive wear resistance and longer wear life in severe service (e.g., rock excavation, primary crushing, heavy mining components) and you can accommodate more restrictive welding/forming procedures and higher material cost. - Choose NM360 if you need a balance of abrasion resistance with better weldability, formability, and lower initial cost—suitable for conveyors, truck bodies, chutes, and medium-wear parts.

Concluding note When specifying either grade, request supplier mill certificates detailing chemical composition and hardness, require welding procedure qualification (PQR/WPS) for the intended thickness and service conditions, and consider designing wear parts for replaceability to optimize life-cycle cost.