Changing the surface condition of a wear-resistant steel plate - either by mechanical treatment, coating, or diffusion hardening - can extend service life by 2× to 5× in abrasive environments. But the right method depends on the wear mechanism: impact, sliding abrasion, erosion, or metal-to-metal galling each demand a different solution.
This guide covers four mainstream surface treatment routes, compares their performance against standard quench-hardened NM-series plates (GB/T 24186-2022), and helps you match the process to the application.
Key Takeaways
- TD (Thermal Diffusion) carbide coating - ideal for tooling and dies (VC layer, 3000–4000 HV). Not a bulk plate treatment for heavy wear applications.
- Quench-hardened wear plates (NM360–NM600) - the industrial workhorse. Through-thickness hardness from 330 to ≥570 HBW. No coating required.
- Hardfacing overlay - chromium carbide deposits (55–65 HRC) on a mild substrate. Best for combined impact + abrasion where through-hardness is not needed.
- Shot blasting - surface preparation only. Removes scale and rust; improves coating adhesion. Not a wear-resistance treatment on its own.
- Selecting the right combination of base plate grade + surface treatment gives far better cost/performance than chasing maximum hardness alone.
1. Why Surface Treatment Matters for Wear Plates
A plain carbon steel plate loses material quickly under abrasive slurry, sliding contact, or repeated impact. The cheapest path to longer life is picking a harder base material - NM400 or NM500 from the quench-and-tempered family, for instance. But hardness alone does not solve every problem. Galling in metal-forming dies, corrosion-assisted wear in mining chutes, or erosion in pneumatic conveying lines often need a tailored surface layer on top of a tough core.
Surface treatments do two things: they decouple surface properties (hardness, friction, corrosion resistance) from bulk properties (strength, toughness, weldability), and they let you upgrade an existing component without replacing the entire plate. The table below shows what each method changes and what it leaves alone.
| Treatment Method | What It Changes | What Stays the Same |
|---|---|---|
| Quench & temper (NM plate) | Bulk hardness, full thickness | - |
| TD carbide coating | Surface 2–15 μm only | Core toughness and ductility |
| Hardfacing overlay | Surface 3–8 mm, 55–65 HRC | Base plate (mild steel) |
| Shot blasting | Surface roughness and cleanliness | Hardness and microstructure |
| Induction hardening | Surface 2–6 mm, localised | Core remains soft and tough |
2. GB/T 24186 NM-Series Wear-Resistant Steel Plates - The Baseline
Before discussing add-on treatments, it is worth understanding what a standard Chinese wear plate already delivers. The NM ("Nai Mo" - wear-resistant) series is covered by GB/T 24186-2022 (High strength abrasion resistant steel plate, sheet and strip for construction machinery).
These are low-alloy quenched-and-tempered steels. Their wear resistance comes from martensitic microstructure through the full thickness - no coating involved.
| Grade | Brinell Hardness (HBW) | Tensile Strength, min (MPa) | Yield Strength, min (MPa) | Typical Thickness (mm) |
|---|---|---|---|---|
| NM360 | 330–390 | ≥950 | ≥800 | 8–80 |
| NM400 | 360–440 | ≥950 | ≥800 | 8–80 |
| NM450 | 420–480 | ≥1050 | ≥900 | 8–60 |
| NM500 | ≥470 | ≥1100 | ≥1000 | 8–50 |
| NM550 | ≥530 | - ¹ | - ¹ | 8–40 |
| NM600 | ≥570 | - ¹ | - ¹ | 8–30 |
These plates are the starting point for most wear applications in mining, cement, steel mills, and construction machinery. If your current plate is NM400 and it lasts six months, jumping to NM500 might give you nine - no surface treatment needed, just a grade change. The remaining sections discuss what to add when the NM series alone is not enough.
3. Thermal Diffusion (TD) Carbide Coating - For Tooling, Not Bulk Plate
The TD (Thermal Diffusion) process - also called Toyota Diffusion Process - was developed by Toyota Central R&D Labs in Japan. It immerses steel workpieces in a borax-based molten salt bath at 850–1050 °C containing carbide-forming elements (vanadium, niobium, chromium, titanium). Carbon and nitrogen atoms from the steel substrate diffuse outward and react with the bath species, forming a dense carbide layer 2–15 microns thick. Vanadium carbide (VC) is the most widely applied variant, reaching 3000–4000 HV - among the hardest coatings available for tool steels.
- Reality check. TD is a die/mold treatment, not a plate treatment. You use TD on a cold-forming punch or a deep-drawing die made of tool steel (SKD11, D2, Cr12MoV). You do not run a 6-meter NM500 wear plate through a TD salt bath. The original article on this page conflated mold surface treatment with bulk wear plate surface treatment - they are different domains. We have corrected this here.
- Where TD shines:
- Cold stamping and drawing dies - VC coating eliminates galling (also called "pick-up" or "scoring") on mild steel and coated sheet metal.
- Powder compaction tooling - the hard, low-friction surface resists abrasive wear from ceramic and metal powders.
- Plastic injection mold components - wear resistance against glass-filled resins.
Where it does not apply: heavy sliding abrasion on bulk wear plates in chutes, crusher liners, or excavator buckets. For those, hardfacing or higher NM grades are the correct route.
4. Hardfacing Overlay - Adding a Wear Skin to Mild Steel
Hardfacing deposits a chromium-carbide-rich alloy onto a mild steel (Q235 / S235JR) substrate by welding - either manual stick, MIG/MAG, or submerged arc. The resulting layer is 3–8 mm thick with a hardness of 55–65 HRC (roughly 550–700 HBW). Because the base plate stays soft, the composite plate can absorb impact without cracking while the surface resists abrasion.
This is a widely used construction in mining and cement plants: a 6+4 or 8+6 configuration, meaning 6 mm of Q235 base + 4 mm of hardfacing, or 8 mm + 6 mm. The overlay chemistry typically contains 25–35% chromium and 3–5% carbon, producing primary M7C3 carbides in a eutectic matrix.
| Property | Hardfacing Overlay Plate | Quench-Hardened NM500 |
|---|---|---|
| Surface hardness | 55–65 HRC (~550–700 HBW) | ≥470 HBW |
| Through-thickness uniformity | No - soft base, hard skin | Yes - martensitic throughout |
| Impact resistance | Good (base absorbs energy) | Moderate (brittle at high hardness) |
| Weldability on site | Good (weld on mild side) | Requires preheat + low-H electrodes |
| Max service temperature | ~500 °C (carbide stability limit) | ~250 °C (tempering limit) |
| Relative cost (per m², 10 mm) | ~1.5× | 1.0× (baseline) |
5. Shot Blasting and Mechanical Surface Preparation
Shot blasting propels steel grit or shot at high velocity onto the plate surface. It removes mill scale, rust, and light contaminants, leaving a uniform profile (typically Sa 2½ per ISO 8501-1). This improves the adhesion of subsequent paint or rubber lining - which matters in corrosive slurry environments where a wear plate is also exposed to acid or salt water.
Shot blasting does not increase the hardness of the steel. The peening effect can introduce shallow compressive residual stress (beneficial for fatigue life), but the depth is typically less than 0.2 mm. For surface preparation before coating, it is essential. For wear resistance, it is irrelevant - grade selection and hardfacing do the real work.
6. Induction and Flame Surface Hardening - Localised Solutions
For large structural wear parts where a full NM plate replacement is impractical - large gears, crane wheels, rail heads - induction or flame hardening can create a hard case (2–6 mm deep, 50–60 HRC) on a medium-carbon steel (0.40–0.50% C). The process heats the surface rapidly above the austenitizing temperature, then quenches it, leaving a martensitic case over a tough, ductile core. It is fast and energy-efficient but only practical on parts that can be rotated or scanned under the induction coil.
7. Selection Logic - Which Treatment for Which Wear Mechanism?
| Wear Mechanism | Typical Application | Recommended Approach |
|---|---|---|
| Low-stress sliding abrasion | Chute liners, hoppers | NM400–NM500 plate, no coating |
| High-stress grinding abrasion | Crusher liners, mill liners | Hardfacing overlay or NM550–NM600 |
| Heavy impact + abrasion | Excavator buckets, truck beds | Hardfacing composite plate (6+4 or 8+6) |
| Metal-to-metal galling | Cold-forming dies, punches | TD carbide coating on tool steel |
| Erosion (particle-laden flow) | Pneumatic conveying elbows | Hardfacing or ceramic-lined pipe; NM500 for flat sections |
| Corrosion + abrasion | Wet scrubbers, chemical chutes | Stainless overlay or rubber-lined NM plate |
| Fatigue + surface wear | Gear teeth, rail heads | Induction/flame surface hardening |
Frequently Asked Questions
What is the TD (Thermal Diffusion) process for wear-resistant steel?
The TD process, originally developed by Toyota in Japan, creates a hard vanadium carbide (VC) layer 2–15 microns thick on steel surfaces via a borax-based salt bath at 850–1050 °C. The resulting coating reaches 3000–4000 HV. It is primarily a mold and tooling treatment, not a bulk plate treatment for heavy wear plates like those used in mining chutes or crusher liners.
What is the hardness range for Chinese NM wear-resistant steel plates?
Per GB/T 24186-2022, the Brinell hardness ranges are: NM360: 330–390 HBW; NM400: 360–440 HBW; NM450: 420–480 HBW; NM500: ≥470 HBW; NM550: ≥530 HBW; NM600: ≥570 HBW. These plates are delivered in quenched-and-tempered condition. Their wear resistance comes from through-thickness martensitic microstructure, not from a surface coating.
Can shot blasting improve the wear life of steel plates?
Shot blasting cleans and profiles the surface - it removes mill scale, rust and contaminants, improving coating adhesion and surface uniformity. It does not measurably increase hardness or wear resistance. For actual wear life improvement, select the right NM grade or add a hardfacing overlay.
What is the difference between hardfacing overlay and quench-hardened wear plate?
Quench-hardened NM plates achieve uniform through-thickness hardness via heat treatment (360–600 HBW). Hardfacing overlay deposits a chromium-carbide-rich layer (55–65 HRC, about 550–700 HBW) onto a mild steel substrate by welding. Hardfacing gives a harder surface but the soft base beneath can deform under heavy point loads. Quench-hardened plates offer uniform properties throughout, making them better where consistent wear depth is expected.
Does Gnee Steel supply surface-treated wear-resistant plates?
Yes. Gnee Steel (Tianjin) Co., Ltd. supplies NM360–NM600 quench-hardened wear-resistant steel plates in thicknesses from 6 mm to 100 mm, plus hardfacing composite plates. We also provide cutting, beveling and pre-blasting preparation. Contact our team for a quotation with your required grade, dimensions and quantity.
References
- GB/T 24186-2022 - High strength abrasion resistant steel plate, sheet and strip for construction machinery. National Standard of China.
- NM400, NM450, NM500 grade specifications and hardness data. Zinsine Materials Database.
- TD Processing (Toyota Diffusion Process) - Carbide Coating Principles. Welleshaft / IBCCoatings.
- Vanadizing / Thermal Diffusion (TD) Process overview. BDI Metal.
- Gnee Steel product catalogue - wear-resistant steel plates.
All standards and figures above have been verified against the cited sources as of August 2026. If you spot an error, contact us so we can correct it.
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Disclaimer: This article is provided for general information and does not constitute engineering advice or a product warranty. Technical data reflects the cited standards and sources at the time of writing; always confirm final properties and compliance with the applicable specification for your project. Gnee Steel (Tianjin) Co., Ltd. makes no representation regarding the accuracy of third-party references. Address: No.4-1114, Beichen Building, Beicang Town, Beichen District, Tianjin, China.
