NM400 vs NM450 – Composition, Heat Treatment, Properties, and Applications
แบ่งปัน
Table Of Content
Table Of Content
Introduction
NM400 and NM450 are widely used high-strength abrasion-resistant (AR) steels specified for applications where surface wear dominates component life: buckets, chutes, liners, crushers, and heavy-duty ground-engaging tools. Engineers and procurement professionals frequently decide between them based on trade-offs among wear resistance, toughness, weldability, manufacturability, and cost. Typical decision contexts include whether a higher upfront material cost for greater abrasion resistance is justified by longer service life, or whether better impact toughness and ease of fabrication are more important.
The principal practical distinction between these two grades is the level of designed wear resistance: NM450 is specified and processed to deliver higher hardness and improved abrasion resistance than NM400, whereas NM400 is optimized for a more balanced strength–toughness–fabrication profile. Because both grades target similar product families (plates, liners, and sections), they are commonly compared when optimizing component life, repair intervals, and total cost of ownership.
1. Standards and Designations
NM-type steels are most commonly encountered in national and regional standards for abrasion-resistant carbon/alloy steels rather than international tool or stainless steel standards.
- GB (China): NM400, NM450 appear in Chinese standards covering wear-resistant steels (often referenced in procurement and mill certificates).
- EN (Europe): Equivalent products are marketed under names like AR400 / AR450 or as EN 10029/10051 families; direct one-to-one correspondence requires mill certificate checks.
- JIS (Japan) / ASTM/ASME (USA): These standards provide separate families for abrasion-resistant steels and high-hardness plates (e.g., ASTM A611, A514, or AR400 equivalents), but again, grade names differ — check chemistry and mechanicals rather than name only.
Classification: NM400 and NM450 are carbon/alloy abrasion-resistant steels, often considered within the broader high-strength low-alloy (HSLA) and quenched/tempered product families rather than tool steels or stainless grades.
2. Chemical Composition and Alloying Strategy
Below is a qualitative comparison of the common chemical design approaches for each grade. Exact compositions vary by mill and must be confirmed by material certificates for any critical application.
| Element | NM400 (typical design intent) | NM450 (typical design intent) |
|---|---|---|
| C (Carbon) | Low–moderate carbon to balance hardness and toughness | Moderate carbon, typically higher than NM400 to assist higher hardness |
| Mn (Manganese) | Moderate Mn for strength and hardenability | Moderate–elevated Mn to support hardenability and strength |
| Si (Silicon) | Small addition for deoxidation and strength | Similar to NM400; controlled level |
| P (Phosphorus) | Kept low (impurity control) | Kept low |
| S (Sulfur) | Low, controlled (inclusions minimized) | Low, controlled |
| Cr (Chromium) | May be present in small amounts to improve wear | Often slightly higher or controlled for hardenability and wear resistance |
| Ni (Nickel) | Typically minimal or absent | Minimal or controlled if toughness enhancement is targeted |
| Mo (Molybdenum) | May be used in small amounts to increase hardenability | Used in some recipes to improve hardenability and tempering resistance |
| V, Nb, Ti (microalloying) | Possible microalloy additions to refine grain size and improve toughness | May include microalloying to improve strength and toughness at higher hardness |
| B (Boron) | Not typically a main design feature but can be used in trace amounts to increase hardenability | Trace additions possible in some mills |
| N (Nitrogen) | Controlled to avoid embrittlement | Controlled |
How alloying affects properties - Carbon and manganese are primary levers for hardness and hardenability. Higher carbon yields higher achievable hardness for a given heat treatment but tends to reduce ductility and complicate welding. - Small additions of Cr and Mo increase hardenability and abrasive wear resistance and improve tempering resistance. - Microalloying elements (V, Nb, Ti) refine grain size and raise yield strength without excessive carbon, improving toughness at elevated hardness levels. - Sulfur and phosphorus are controlled because they reduce toughness and can affect weldability.
3. Microstructure and Heat Treatment Response
Typical manufacturing routes include controlled rolling/thermo-mechanical processing, quenching and tempering (Q&T), and in some cases accelerated cooling to produce a bainitic or tempered martensitic structure.
NM400 - Produced to achieve a tempered martensite or fine bainite/martensite matrix depending on processing. - Thermo-mechanical rolling with controlled cooling yields a refined ferrite–pearlite or bainitic structure with local hard phases; Q&T variants produce tempered martensite for higher hardness. - Response to heat treatment: moderate hardenability allows attainment of target hardness with conventional quenching cycles; tempering restores toughness.
NM450 - Processed to achieve higher hardness levels; typical microstructure targets finer, higher-strength bainite or tempered martensite with a greater proportion of hard constituents. - Thermo-mechanical rolling with more aggressive cooling or Q&T designed for higher hardenability is common. - Response to heat treatment: requires slightly different thermal cycles to avoid excessive brittleness while maximizing hardness; tempering is critical to balance strength and toughness.
Practical consequences - NM450’s microstructure is pushed toward higher hardness and wear-resistant phases; this improves abrasive life but requires stricter control of heat treatment and cooling rates to maintain toughness. - Normalizing or stress-relief cycles are used differently: over-tempering or improper cooling can reduce wear performance; insufficient tempering can leave brittle microstructures.
4. Mechanical Properties
Provided as qualitative comparisons; exact mechanical properties depend on product form, thickness, and mill heat treatment.
| Property | NM400 | NM450 |
|---|---|---|
| Tensile strength | High (service-oriented) | Higher than NM400 |
| Yield strength | High | Higher than NM400 |
| Elongation (ductility) | Moderate — good balance | Slightly lower than NM400 at comparable hardness |
| Impact toughness | Better balance between toughness and hardness | Can be lower if hardness maximized; manufacturers often tailor for adequate toughness |
| Hardness (surface/hardness rating) | Designed for ~400 HB hardness-class performance (designation basis) | Designed for ~450 HB hardness-class performance (designation basis) |
Why these differences occur - NM450 targets higher hardness for abrasion resistance by increased carbon, alloying, or different processing; that raises tensile and yield strengths but can reduce elongation and impact toughness if not balanced with alloying and thermal processing. - NM400 is often chosen where a compromise between toughness and wear resistance is required, especially where impact and gouging are significant.
5. Weldability
Weldability is governed largely by carbon equivalent and microalloying content. Two common empirical formulas used to assess risk of hydrogen-assisted cold cracking and hardening of the heat-affected zone are:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation (qualitative) - Higher carbon, higher Mn, and additions of Cr/Mo/V raise $CE_{IIW}$ and $P_{cm}$, indicating greater hardenability and a higher risk of HAZ hardening and cracking. NM450 typically shows higher indices than NM400, implying tighter preheat, interpass temperature control, and post-weld heat treatment requirements. - Microalloying (Nb, V, Ti) increases strength but can increase hardenability; however, controlled levels and proper welding procedures mitigate cracking risk. - Practical recommendations: use low-hydrogen consumables, preheat thicker sections or high $CE$ steels, control interpass temperatures, and consider PWHT or local heat input management when welding NM450 in heavy sections.
6. Corrosion and Surface Protection
NM400 and NM450 are non-stainless carbon/alloy steels; corrosion resistance is limited and not intrinsic to their designation.
- Non-stainless protection: common surface protections include painting, abrasion-resistant coatings, thermal spray overlays, and hot-dip galvanizing (where appropriate for the service environment). In most abrasive environments, sacrificial coatings will be consumed quickly; frequently, wear liners are used as replaceable sacrificial elements.
- When stainless-like corrosion indexes are considered, a PREN formula is used for stainless grades; it is not applicable to NM grades:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Clarification: NM grades are not designed for corrosion resistance; PREN and stainless criteria are irrelevant unless the alloy chemistry intentionally includes substantial Cr/Mo and N (in which case the material would not be typical NM400/NM450).
7. Fabrication, Machinability, and Formability
- Cutting: Higher hardness of NM450 increases tool wear during thermal cutting and mechanical machining compared with NM400. Plasma and oxy-fuel cutting parameters must be adjusted; preheat and slower cutting can be required to avoid cracking.
- Bending/forming: Ductility reductions at higher hardness mean NM400 is generally easier to form; NM450 requires tighter bend radii control and potentially preheating or local heat treatment for severe forming operations.
- Machinability: Both are more difficult to machine than mild steels; NM450 is typically the more challenging due to higher hardness and abrasive carbides. Use of carbide tooling, rigid setups, and conservative parameters is standard.
- Finishing: Grinding and surface finishing for NM450 demand more frequent tool dressing and attention to heat generation to avoid surface tempering.
8. Typical Applications
| NM400 (common uses) | NM450 (common uses) |
|---|---|
| Truck and loader bucket liners where impact and moderate abrasion occur | High-wear liners, chutes, and screens subjected to severe abrasive wear where service life is prioritized |
| Conveyor skids, wear plates in moderate abrasion environments | Grinding mill liners, heavy-duty crusher jaws, and applications where cutting/grinding abrasion dominates |
| Parts requiring better balance of toughness and wear (e.g., rock-breakers) | Parts where maximum abrasion resistance is required and replacement intervals must be minimized |
Selection rationale - Choose NM400 when parts see combined impact and abrasion, require better ductility and weldability, or when lower total material cost with more frequent replacement is acceptable. - Choose NM450 when abrasive wear is the dominant failure mode and maximizing time between service or replacements outweighs additional material and fabrication costs.
9. Cost and Availability
- Relative cost: NM450 typically commands a premium over NM400 due to higher alloying, more stringent processing, and the need for tighter quality control. Exact price differences vary with market, thickness, and mill.
- Availability: Both grades are widely produced in plate and liner forms by major mills, but availability for specific thicknesses and surface conditions can vary regionally. NM400 is often more readily available in a broader range of thicknesses; NM450 may be stocked in standard plate sizes and certain thickness bands.
Product form considerations - Plate, cut-to-size liners, welded assemblies, and fabricated components are common product forms. Lead times and custom heat treatments increase with complexity.
10. Summary and Recommendation
Summary table (qualitative)
| Characteristic | NM400 | NM450 |
|---|---|---|
| Weldability | Better (lower hardenability on average) | More demanding (higher hardenability) |
| Strength–Toughness balance | Balanced (good ductility and toughness) | Higher strength and hardness; toughness must be engineered |
| Cost | Lower (generally) | Higher (generally) |
Recommendation - Choose NM400 if: - The service includes a significant combination of impact and abrasion. - Fabrication, weldability, and ductility are important to reduce repair complexity. - Lower material cost and easier field welding are priorities.
- Choose NM450 if:
- Abrasive wear is the dominant failure mode and maximizing service life is more valuable than initial material and fabrication cost.
- The design and fabrication process can accommodate stricter welding and heat-treatment control.
- Parts are replaceable liners or components where higher hardness directly translates to fewer interventions.
Final considerations - Always verify mill certificates for actual chemistry and mechanical properties before specifying or accepting material. - For critical components, request hardness maps, heat-treatment records, and, if needed, impact toughness data at the intended service temperature. - Pilot trials, field trials, or wear testing under representative service conditions often provide the best basis for final grade selection and total cost of ownership calculations.