NM400 vs NM450HB – Composition, Heat Treatment, Properties, and Applications

Introduction

NM400 and NM450HB are two wear-resistant structural steel designations commonly considered for components exposed to sliding and abrasive wear — such as liners, buckets, hoppers, and crusher parts. Engineers, procurement managers, and manufacturing planners routinely weigh trade-offs among wear resistance, strength, weldability, and cost when choosing between them. Typical decision contexts include balancing maximum service life (abrasion resistance) against fabrication complexity (welding, forming) and overall component toughness under impact.

The principal practical difference between these grades is that NM450HB is engineered and delivered to a higher hardness (and therefore generally higher strength and wear resistance) than NM400, which produces different microstructures and processing demands. Because both are designed as high-strength, wear-resistant steels (often produced by controlled rolling and quench-and-temper or direct quench processes), they are compared frequently when specifying parts for severe service.

1. Standards and Designations

• Common standards and designations where NM-type steels appear:
• GB/T (China): NM series (e.g., NM400). These are typically wear-resistant steels specified by nominal hardness.
• JIS (Japan) and EN (Europe) provide equivalent concepts (hardness-based wear steels) but use different designations (e.g., AR (abrasion-resistant) steels, HBW hardness classes).
• ASTM/ASME: no single "NM" designation; ASTM standards cover nomenclature for quenched-and-tempered steels, abrasion-resistant steels, or limit mechanical properties by spec.
• Individual producers may label proprietary grades with similar names (e.g., HB numbering indicates target Brinell hardness).
• Material class: Both NM400 and NM450HB are high-strength, wear-resistant structural steels — not stainless steels nor conventional tool steels. They are typically classified as quenched-and-tempered or heat-treated carbon–manganese microalloyed steels (a subset of HSLA/wear steels).

2. Chemical Composition and Alloying Strategy

The following table describes the typical alloying elements and their relative presence in NM400 and NM450HB. Exact ranges vary by producer and standard; consult mill certificates for precise chemistry.

Element	NM400 (typical)	NM450HB (typical)	Comment
C	Low–medium	Low–medium (may be similar or slightly lower)	Carbon provides base strength/hardness but is kept controlled to preserve weldability.
Mn	Medium	Medium–high	Manganese promotes hardenability and tensile strength; higher Mn aids wear resistance.
Si	Low–medium	Low–medium	Silicon aids deoxidation and can contribute to strength.
P	Trace	Trace	Kept minimized for toughness and weldability.
S	Trace	Trace	Low sulfur is preferred to avoid embrittlement and improve toughness.
Cr	Trace–low	Trace–low	Small Cr additions can improve hardenability and temper resistance.
Ni	Trace–low	Trace–low	Rare in basic NM steels; used by some mills to improve toughness.
Mo	Trace–low	Trace–low	Mo increases hardenability and tempering resistance if present.
V, Nb, Ti	Microalloy (trace)	Microalloy (trace)	Microalloying refines grain and strengthens via precipitation; used selectively.
B	Trace (rare)	Trace (rare)	Very small B can markedly increase hardenability when controlled.
N	Trace	Trace	Nitrogen controlled to avoid embrittlement.

How alloying affects properties: - Carbon and manganese are primary contributors to hardness and hardenability. Controlling carbon is a balance: sufficient for strength but limited for weldability. - Microalloying elements (V, Nb, Ti) refine the prior austenite grain size and improve the yield strength–toughness balance without excessive carbon. - Small additions of Cr and Mo (when present) improve hardenability and temper resistance, aiding retention of hardness in thicker sections.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses:

• NM400:
• Often supplied after quench-and-temper or controlled rolling + tempering. Microstructure commonly consists of tempered martensite, bainite, or a tempered martensite/bainite mix depending on cooling rate and alloying.
• With moderate hardenability, thicker sections may show mixed microstructures (bainite + martensite), which helps toughness.

• Normalizing followed by tempering can produce a homogeneous microstructure for improved toughness at somewhat lower hardness.

• NM450HB:

• Targeted for higher Brinell hardness; achieved by higher hardenability (through alloying and processing) and more aggressive quench-and-temper schedules or direct quenching.
• Microstructure tends to be a higher fraction of martensite or very fine bainite. The finer the martensitic structure and the more homogeneous the tempering, the better the toughness for the same hardness.
• Thermo-mechanical controlled processing (TMCP) and precise heat treatment are more critical to achieve the higher hardness while preserving acceptable toughness.

Heat-treatment effects: - Normalizing refines grains and improves toughness but reduces peak hardness compared with quench-and-temper. - Quenching and tempering raises hardness and strength (NM450HB often uses a higher quench severity or alloying to reach target HB). - TMCP can produce superior strength–toughness balance and reduce required heat treatment severity for high hardness grades.

4. Mechanical Properties

The following table gives qualitative and industry-conventional hardness targets and typical mechanical behavior. For exact values, reference the mill certificate and relevant standard.

Property	NM400	NM450HB	Notes
Tensile Strength	High	Higher	NM450HB is engineered for increased tensile strength consistent with higher hardness.
Yield Strength	High	Higher	Yield typically rises with hardness; NM450HB shows higher yield.
Elongation (%)	Moderate	Lower (relative)	Increased hardness tends to reduce ductility; careful heat treatment can mitigate loss.
Impact Toughness	Good–variable	Lower–variable	Higher hardness can reduce impact energy, especially at low temperatures; specification-dependent.
Hardness (Brinell)	Nominally ~400 HB class	Nominally ~450 HB class	Grades are often named for target HB, so NM400 ≈ 400 HB class and NM450HB ≈ 450 HB class.

Why differences occur: - Higher hardness (NM450HB) indicates a microstructure with more martensite/finer bainite and greater resistance to plastic deformation — hence higher wear resistance and strength but reduced ductility/toughness relative to NM400 under comparable processing.

5. Weldability

Weldability is influenced by carbon content, alloying, section thickness, and microalloying. Common predictive formulas:

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

• Pcm (for more conservative assessment of weldability): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - NM450HB, with higher hardenability and often more microalloying, generally has higher $CE_{IIW}$ and $P_{cm}$ than NM400, indicating a greater tendency to form hard martensitic HAZ microstructures and thus a higher susceptibility to cold cracking unless preheat and controlled interpass temperatures are used. - Preheat, controlled filler metal selection (matching or slightly lower hardness filler), and post-weld heat treatment (PWHT) can mitigate weld HAZ cracking risk. - For thick sections or critical structures, welding procedure qualification and hydrogen control are essential for NM450HB.

6. Corrosion and Surface Protection

• Non-stainless nature: Neither NM400 nor NM450HB is corrosion-resistant by composition. Corrosion resistance must be achieved by coatings or cathodic protection.
• Typical protection methods:
• Galvanizing: Possible depending on component geometry and service; note that hot-dip galvanizing involves thermal exposure and may affect heat-treated properties unless the part is coated after final heat treatment.
• Paint systems: Epoxy/polyurethane coatings for atmospheric protection.
• Rubber or polymer overlays: For abrasive applications combined with corrosive environments.
• PREN (pitting resistance equivalent number) is not applicable to these grades because PREN is used for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use corrosion protection strategies tailored to combined abrasion-corrosion environments (e.g., select coatings or lining systems compatible with abrasive wear).

7. Fabrication, Machinability, and Formability

• Machinability:
• Harder grades (NM450HB) are more difficult to machine; tool wear increases and cutting speeds/feed must be reduced. Carbide tooling and stable machine setups are recommended.
• NM400 is comparatively easier to machine, but still more challenging than mild steel.
• Formability and bending:
• Higher hardness reduces formability. NM450HB typically requires larger bend radii, lower forming strain, or hot forming / localized annealing to avoid cracking.
• Cold bending of NM400 is easier but still requires attention to springback and edge cracking.
• Cutting and thermal processing:
• Plasma/oxy-fuel cutting and waterjet are common; heat-affected zones from thermal cutting can introduce brittle microstructures—post-cut grinding or gouging and stress relief may be needed.
• Finishing:
• Grinding and shot-peening can be effective for surface preparation and to extend fatigue life; finish requirements depend on service.

8. Typical Applications

NM400 (common uses)	NM450HB (common uses)
Bucket and shovel liners for general mining and construction	Crusher jaws, cone liners, and components subject to severe abrasive wear and concentrated loads
Chute, hopper, and conveyor wear plates in medium-abrasion environments	High-wear mill and comminution equipment where maximum service life is required
Ground-engaging tools with moderate impact and abrasion	Critical components where extended wear life justifies higher material and processing cost
Wear strips, liners, and skid plates where some ductility is needed	Severe service plates and liners in heavy mining and aggregate industries

Selection rationale: - Choose NM400 when a balance of wear resistance, toughness, and easier fabrication is required for medium-to-high abrasion environments. - Choose NM450HB when maximum abrasion resistance and higher strength are primary drivers and fabrication complexity and cost increases are acceptable.

9. Cost and Availability

• Relative cost: NM450HB typically costs more than NM400 on a per-ton basis due to additional heat treatment, tighter processing control, and increased alloying/processing demands.
• Availability:
• Both grades are commonly available in plate and fabricated forms from major mills, but NM450HB may have longer lead times or minimum order quantities depending on supplier inventory and plate thickness.
• Specialty thicknesses or certified mill-heat treatment conditions can increase lead time for either grade.

10. Summary and Recommendation

Summary table (qualitative):

Criterion	NM400	NM450HB
Weldability	Good (easier)	Fair–difficult (requires controls)
Strength–Toughness balance	Good	Higher strength; lower ductility (relative)
Cost	Lower	Higher
Abrasion resistance	High	Very high

Recommendations: - Choose NM400 if: - You require a cost-effective, high-wear material with better fabrication and welding latitude. - The service environment involves mixed abrasion and impact where toughness and ductility are important. - Shorter lead-times and easier forming/machining are priorities.

• Choose NM450HB if:
• Maximizing wear life is the primary objective, and higher hardness/strength will materially reduce downtime or replacement costs.
• The design can accommodate more stringent welding, preheat, and fabrication controls (or machining is minimized).
• The increased upfront material and processing cost is justified by the longer in-service life.

Final note: Always consult the mill material certificate, supplier technical data, and perform application-specific validation (lab wear tests, welded coupon testing, and prototype trials) before committing to a grade for critical components. Welding procedures, heat treatment records, and post-installation inspection plans are particularly important when moving from NM400 to higher-hardness NM450HB to ensure structural integrity and predictable service life.