420 vs 440C – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
420 and 440C are two widely used martensitic stainless steels that appear frequently in procurement lists for parts requiring a balance of hardness, wear resistance, and corrosion performance. Engineers, procurement managers, and manufacturing planners commonly face a tradeoff between cost, machinability, and the hardness/wear capability required for a part: the right choice depends on service loads, surface finish, and expected corrosive environment.
The principal practical distinction between these grades is their alloying strategy: one is a lower‑carbon martensitic stainless with moderate chromium providing corrosion resistance and reasonable toughness; the other is a high‑carbon, high‑chromium martensitic stainless designed to form a significant population of hard chromium carbides for superior wear resistance and higher attainable hardness. That difference drives their divergent heat‑treatment behavior, mechanical properties, weldability, and application domains.
1. Standards and Designations
- 420:
- Common designations: UNS S42000, EN X46Cr13 (or X20Cr13 depending on variant), JIS SUS420J1 / SUS420J2.
- Typical standards: ASTM A276 (stainless steel bars, bars and shapes), ASME, EN, JIS.
- Category: Martensitic stainless steel (stainless tool/knife steel).
- 440C:
- Common designations: UNS S44004, EN X105CrMo17 (historical), JIS SUS440C.
- Typical standards: AMS, ASTM A582 / A666 (some product forms), EN, JIS.
- Category: High carbon martensitic stainless/tool steel.
420 is generally specified where moderate corrosion resistance and moderate hardness suffice; 440C is specified where higher hardness, wear resistance, and edge retention are required.
2. Chemical Composition and Alloying Strategy
| Element | 420 (typical ranges, wt%) | 440C (typical ranges, wt%) |
|---|---|---|
| C (Carbon) | 0.15 – 0.40 | 0.95 – 1.20 |
| Mn (Manganese) | ≤ 1.00 | ≤ 1.00 |
| Si (Silicon) | ≤ 1.00 | ≤ 1.00 |
| P (Phosphorus) | ≤ 0.04 | ≤ 0.04 |
| S (Sulfur) | ≤ 0.03 | ≤ 0.03 |
| Cr (Chromium) | 12.0 – 14.0 | 16.0 – 18.0 |
| Ni (Nickel) | ≤ 0.60 | ≤ 0.50 |
| Mo (Molybdenum) | trace – ≤ 0.60 (variant) | trace – ≤ 0.75 (some specs) |
| V, Nb, Ti, B, N | typically trace/none | typically trace/none |
Notes: - Values above are representative ranges from common specifications and product datasheets; exact composition depends on standard and producer. - 440C’s substantially higher carbon and higher chromium content promote a higher volume fraction of hard chromium carbides (primarily M23C6/M7C3 types in practical microstructures), increasing wear resistance and achievable hardness. - 420’s lower carbon produces fewer carbides and a more ductile martensite matrix after quench-and-temper, improving toughness and machinability versus 440C.
How alloying affects behavior: - Carbon controls hardenability and maximum attainable hardness after quench/temper; higher carbon → higher hardness but reduced toughness and weldability. - Chromium provides corrosion resistance by forming a passive oxide film; higher Cr generally improves resistance, but heavy carbide precipitation can locally deplete matrix chromium. - Alloying elements such as Mo (when present) can improve pitting resistance and hardenability; Mn and Si are processing and deoxidation elements with modest microstructural effects.
3. Microstructure and Heat Treatment Response
Both grades are martensitic stainless steels in common heat‑treated conditions, but their microstructures differ significantly:
- 420:
- Microstructure after quench: predominantly martensite with relatively low carbide volume fraction; carbides are finer and fewer.
- Heat treatment: austenitize (typical range ~980–1030 °C depending on specification), quench (oil/air depending on section size), temper to required hardness (tempering between ~150–600 °C). Maximum practical hardness is limited by carbon content (often up to ~48–52 HRC for higher‑C variants).
-
Response: good response to conventional quench & temper cycles; lower retained austenite fraction; tempering improves toughness.
-
440C:
- Microstructure after quench: martensitic matrix with a significant volume fraction of chromium‑rich carbides. Carbide distribution is a major contributor to wear resistance.
- Heat treatment: austenitize typically in the range ~1010–1070 °C, quench (oil or air for small sections), temper depending on target hardness. Cryogenic treatments are sometimes used to reduce retained austenite and convert it to martensite, followed by low‑temperature tempering to stabilize hardness.
- Response: high carbon enables very high as‑quenched hardness but also increases risk of distortion and cracking. Tempering trades hardness for toughness; optimal tempering balances retained hardness versus brittle failure risk.
Normalizing, repeated tempering, or subzero treatments have different outcomes: 440C benefits more from carbide control and cryo treatments to maximize hardness and dimensional stability, while 420 is more forgiving in thermal cycles.
4. Mechanical Properties
| Property | 420 (typical, condition-dependent) | 440C (typical, condition-dependent) |
|---|---|---|
| Tensile strength | Moderate to high after hardening; increases with higher C and martensite fraction | Generally higher maximum tensile strength after hardening due to higher carbon |
| Yield strength | Moderate; dependent on heat treatment | Higher when fully hardened |
| Elongation (ductility) | Higher ductility (annealed or tempered) — better formability | Lower elongation when hardened; can be brittle if over‑tempered |
| Impact toughness | Better toughness relative to 440C (same hardness bracket) | Lower toughness due to carbide population and higher hardness |
| Hardness (HRC) | Typically up to ~48–52 HRC (higher‑C variants approach upper end) | Typically up to ~58–64 HRC in properly hardened and tempered condition |
Qualitative explanation: - 440C attains higher hardness and wear resistance because its higher carbon forms a larger quantity of hard chromium carbides embedded in the martensitic matrix. This raises tensile and compressive strength but reduces toughness and ductility. - 420, with lower carbon and fewer carbides, yields better toughness and machinability but cannot match the edge retention or wear resistance of 440C.
Note: Exact mechanical numbers depend strongly on product form (bar, plate), section size, and precise heat‑treatment parameters. Suppliers’ datasheets should be referenced for design calculations.
5. Weldability
Weldability is primarily influenced by carbon equivalent and hardenability. Two common empirical indices:
-
IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
German Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - 440C’s significantly higher carbon and elevated chromium raise $CE_{IIW}$ and $P_{cm}$, indicating a higher propensity to form hard martensitic microstructures in the heat‑affected zone (HAZ) and therefore a greater risk of cold cracking and hydrogen embrittlement. Preheat, controlled heat input, use of low‑hydrogen consumables, and postweld heat treatment (PWHT) are generally required for trouble‑free welding of 440C. - 420, with lower carbon, has better weldability by comparison but still demands attention: preheat and postweld tempering may be advised for critical applications to avoid hard martensitic HAZ and to relieve residual stresses. - In practice, both grades are not as weldable as austenitic stainless steels; welding is often avoided for critical, high‑hardness 440C components. Fabrication by machining from bar stock is common.
6. Corrosion and Surface Protection
- 420:
- With chromium around 12–14%, 420 provides moderate corrosion resistance in mild atmospheres and to light chemicals. It is commonly used in cutlery and less aggressive environments. Surface finishing (polishing, passivation) improves corrosion resistance.
- For aggressive or marine environments, additional protection such as plating, coating, or specifying a higher‑alloy stainless is recommended.
- 440C:
- Higher chromium nominally improves corrosion resistance potential, but the high carbide volume fraction and carbide precipitation during heat treatment can locally deplete chromium in the matrix and reduce pitting resistance. In neutral to mildly corrosive environments, 440C performs adequately; in highly corrosive or chloride‑bearing environments it is not optimal without surface protection.
- PREN (not usually decisive for these martensitic grades but informative for pitting resistance): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For 420 and 440C, PREN values are modest because Mo and N are low or absent; PREN is more relevant to austenitic/ferritic stainlesses with significant Mo and N.
- Surface protection options for non‑suitable corrosion environments: electroless nickel, chromium plating, PVD coatings, passivation, painting, or specifying a corrosion‑resistant alternative alloy.
7. Fabrication, Machinability, and Formability
- Machinability:
- 420: easier to machine in annealed condition; higher carbon 420HC variants are more challenging but still easier than 440C. Good surface finish is achievable.
- 440C: more difficult to machine due to high hardness and abrasive chromium carbides; recommended to machine in softer annealed condition and finish grind after hardening. Tool wear is higher; use carbide tooling, reduced feeds, and coolant.
- Formability and bending:
- 420: better bendability and forming in annealed state; tempered parts are less ductile.
- 440C: limited forming after hardening; typical practice is to perform forming and machining in annealed condition, then heat‑treat to final hardness.
- Surface finishing:
- 440C can be polished to high luster but requires more grinding/polishing effort post‑hardening. 420 polishes comparatively easily and takes passivation well.
8. Typical Applications
| 420 — Typical Uses | 440C — Typical Uses |
|---|---|
| Cutlery and kitchen knives (entry to mid-level) | High‑end cutlery and knives requiring superior edge retention |
| Surgical instruments and dental tools (some types) | Wear parts: valve balls, seats, bearings, bushings |
| Shafts, spindles, pump components in moderately corrosive media | Precision ball bearings, rollers, cams, wear plates |
| Decorative hardware, fasteners, trim | Cutting tools and dies where stainless corrosion resistance and wear resistance are both required |
Selection rationale: - Choose 420 for applications prioritizing corrosion resistance and toughness at lower cost and when extreme hardness is not required. - Choose 440C for applications where wear resistance, edge retention, and the ability to reach very high hardness are primary requirements, and where post‑treatment processing (grinding, polishing) is acceptable.
9. Cost and Availability
- Relative cost: 440C is typically more expensive per kilogram than 420 due to higher alloy content, tighter process control, and demand in tool/wear markets. Specialized product forms (precision ground bars, pre‑hardened stock) for 440C can carry further premium.
- Availability: Both grades are widely available in bar, sheet, and rod but product forms differ. 420 is ubiquitous in flatware and general-purpose stainless markets. 440C is readily available in precision bar and round stock for tooling and bearing applications but is less common in large plate sizes.
- Procurement tip: Buying in common product sizes and pre‑hardened conditions can reduce lead time and cost; custom heat treatments or additional finishing (grinding/cryogenic treatments) increase purchase price and lead time.
10. Summary and Recommendation
| Attribute | 420 | 440C |
|---|---|---|
| Weldability | Better (lower C) — still requires controls | Worse (high C & Cr) — preheat/PWHT often needed |
| Strength–Toughness tradeoff | Better toughness at moderate strength | Higher maximum hardness & wear resistance, lower toughness |
| Cost | Lower | Higher |
Recommendations: - Choose 420 if: - You need moderate corrosion resistance with reasonable toughness and machinability. - Cost, formability, and easier fabrication/welding are priorities. - The application involves moderate wear or impact loading where extremely high hardness is not required. - Choose 440C if: - Maximum hardness, wear resistance, and edge retention are critical (bearing races, valve seats, high‑end knives). - You can accommodate more complex heat treatment, finishing (grinding, polishing), and tighter welding precautions or prefer machining from pre‑hardened stock. - The service environment is not highly corrosive or you plan to apply surface protection.
Closing note: Both 420 and 440C are useful martensitic stainless options; the choice should be driven by the balance of required hardness/wear resistance versus toughness, ease of fabrication, and corrosion environment. For critical components, specify the heat‑treatment condition and request supplier mechanical test data to ensure the delivered microstructure and properties meet design requirements.