20Cr vs 20CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

20Cr and 20CrMo are two widely used low‑alloy carburizing steels encountered in transmission, automotive, and general machinery components. Engineers and procurement specialists commonly evaluate them for parts that require a wear‑resistant surface combined with a ductile, fatigue‑resistant core (for example, gears, shafts, and pinions). The selection dilemma typically centers on cost and availability versus the need for deeper hardenability and improved core strength in larger or higher‑load components.

The principal metallurgical distinction is the controlled addition of molybdenum in 20CrMo, which increases hardenability and resistance to temper softening compared with the Mo‑free 20Cr family. Because both grades are designed as carburizing steels, they are often compared when specifying case‑hardened components where core mechanical properties, heat‑treatment response, and weldability differ in meaningful ways.

1. Standards and Designations

Common standards and designation families where these steels appear include: - GB/T (China): 20Cr, 20CrMo (carburizing alloy steels) - JIS (Japan): similar carburizing grades exist (e.g., SNCM/SCM families for Mo‑bearing steels) - EN (Europe): approximate equivalents are in the 16MnCr5 / 18CrNiMo7 families (note: direct one‑to‑one matches are rare) - ASTM/ASME: no exact direct names; cross‑references are usually by matching chemical composition and property requirements Classification: both are alloy steels intended for carburizing (case‑hardening) applications — not stainless, tool steel, or HSLA in the modern sense.

Always confirm the exact standard and specification called out in purchase orders, as composition windows and permitted impurity levels vary by standard.

2. Chemical Composition and Alloying Strategy

Typical compositions (approximate ranges by weight %; consult the controlling standard or supplier Certificate of Analysis for exact limits):

Element	Typical 20Cr (wt%)	Typical 20CrMo (wt%)
C	0.17 – 0.24	0.17 – 0.24
Mn	0.25 – 0.65	0.30 – 0.65
Si	0.10 – 0.35	0.10 – 0.35
P	≤ 0.035 (max)	≤ 0.035 (max)
S	≤ 0.035 (max)	≤ 0.035 (max)
Cr	0.50 – 1.10	0.30 – 0.70
Ni	≤ 0.40 (if present)	≤ 0.40 (if present)
Mo	≤ 0.08 (usually minimal)	0.15 – 0.30
V	≤ 0.08 (trace)	≤ 0.08 (trace)
Nb	≤ 0.02 (trace)	≤ 0.02 (trace)
Ti	≤ 0.02 (trace)	≤ 0.02 (trace)
B	≤ 0.001 (trace)	≤ 0.001 (trace)
N	typically low (ppm)	typically low (ppm)

Alloying strategy summary: - Carbon is kept moderate to enable effective carburizing (a low bulk C to accept an enriched surface carbon profile). - Chromium provides hardenability and some temper resistance and contributes to wear and scuffing resistance in the case. - Molybdenum in 20CrMo is the intentional addition: small amounts substantially increase deep hardenability, delay the martensite start temperature, and improve resistance to tempering and over‑tempering in heavy or thick sections. - Microalloying elements (V, Nb, Ti) if present in trace amounts can refine grain size and aid toughness, but they are not primary hardeners in these grades.

3. Microstructure and Heat Treatment Response

Typical microstructures: - As‑rolled or normalized: predominantly ferrite + pearlite (fine pearlite desirable). - After carburizing + quench: surface case forms high‑carbon martensite (often with retained austenite depending on carburizing and quench); core transforms to tempered martensite or bainite depending on hardenability and cooling rate. - After tempering: the case is tempered martensite with carbides; the core is tempered martensite/bainite providing toughness.

How heat treatment routes affect them: - Normalizing refines grain size and homogenizes microstructure; both grades respond similarly. - Carburize + quench + temper: the process is the primary use case. Surface hardness depends on case carbon and quench severity; core toughness depends on alloy hardenability. - Quench & temper (without carburizing): used for some small components; 20CrMo achieves higher core strength for the same tempering due to Mo‑induced hardenability and temper‑resistance. - Thermo‑mechanical processing: grain refinement and controlled rolling can improve toughness for both grades; the effect of Mo remains to support deeper hardening in large sections.

Because molybdenum raises hardenability and slows softening during tempering, 20CrMo produces a tougher, higher‑strength core after identical heat treatment in thicker sections compared to 20Cr.

4. Mechanical Properties

Typical property ranges (after common carburizing + quench & temper processes; approximate):

Property	20Cr (typical)	20CrMo (typical)
Tensile strength (core), MPa	700 – 950	750 – 1000
Yield strength (core), MPa	450 – 700	500 – 800
Elongation (A5, core), %	10 – 18	8 – 16
Impact toughness (Charpy V, core), J	30 – 70	30 – 80
Case hardness (HRC, surface)	58 – 62 (depends on case depth)	58 – 62 (depends on case depth)

Interpretation: - Surface hardness achievable by carburizing is similar for both grades because surface carbon controls case hardness. - 20CrMo generally achieves higher core strength and improved temper resistance (less reduction of strength during tempering) — especially important for larger cross‑sections. This makes 20CrMo preferable where deeper hardenability and higher core properties are required. - Ductility and toughness trade off with strength; specifics depend strongly on heat treatment and section size.

Note: Values above are representative ranges; always verify mechanical property requirements vs. the specific standard or test certificate.

5. Weldability

Weldability is governed by bulk carbon equivalent, alloying elements, and section thickness. Two commonly used predictive indices are:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

and

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both 20Cr and 20CrMo have moderate carbon and low to moderate alloying; calculated $CE$ and $P_{cm}$ are typically in a range that requires preheat and controlled interpass temperatures for thicker sections. - The presence of Mo in 20CrMo raises hardenability and therefore increases the risk of cold‑cracking in the heat‑affected zone (HAZ) relative to 20Cr. Thus, 20CrMo often needs more conservative welding procedures (higher preheat, tempering post‑weld on thick sections). - For thin sections with proper procedure, both are weldable with appropriate filler metals and process control. For critical parts a post‑weld heat treatment (PWHT) is often recommended.

Always compute the relevant $CE$ or $P_{cm}$ for supplier chemistry and follow welding procedure specifications (WPS) accordingly.

6. Corrosion and Surface Protection

Neither 20Cr nor 20CrMo is stainless; corrosion resistance is similar and limited. Typical protection methods: - Surface finishes: painting, powder coating, or conversion coatings. - Galvanizing: possible depending on component geometry and dimensional allowances. - Corrosion inhibitors or lubricants for contact surfaces.

Stainlessness indices such as PREN are not applicable to these non‑stainless alloy steels. If corrosion resistance is a primary requirement, select a stainless or corrosion‑resistant alloy rather than relying solely on coatings.

7. Fabrication, Machinability, and Formability

• Machinability: Both grades in the as‑rolled condition offer fair machinability typical of low‑alloy steels; machinability worsens after heat treatment and carburizing.
• Formability: Forming operations are performed in the low‑carbon, pre‑carburized condition for both grades. 20CrMo may exhibit slightly different formability behavior if microalloying or higher hardenability alters flow stress, but practical differences are small.
• Grinding and finishing: Finished parts (carburized + ground) are comparable; 20CrMo core sections may be harder to grind if tempered to higher strength levels.

For high‑volume manufacturing, consider the downstream effects of added molybdenum on tooling wear and grinding cycle times.

8. Typical Applications

20Cr (typical uses)	20CrMo (typical uses)
Small to medium gears, pinions, and splines (thin sections)	Heavily loaded gears, large pinions, and shafts (thick sections)
Shafts, axles for moderate loads	Automotive crankshafts, heavy-duty transmission gears
Small sprockets, fasteners requiring a carburized case	High‑stress fasteners, studs, and components requiring higher core strength
Agricultural equipment parts	Parts exposed to cyclic loading with larger cross‑sections

Selection rationale: - Choose 20Cr when manufacturing cost and standard carburized performance suffice for thin to medium cross‑section components. - Choose 20CrMo when deeper hardenability, better temper resistance, and higher core strength are required — particularly for larger gears and components subject to heavy fatigue or shock loading.

9. Cost and Availability

• Cost: 20CrMo is generally more expensive than 20Cr due to the molybdenum addition and slightly more complex melting/analysis control.
• Availability: Both are common in markets that use GB/JIS standards; availability by product form (bar, plate, forging, ring) depends on regional mills. Larger cross‑section forgings in 20CrMo may have lead times or minimum order quantities.
• Procurement tip: Specify exact chemical limits and heat‑treatment requirements; request mill test reports (MTRs) and confirm lead times for Mo‑bearing grades.

10. Summary and Recommendation

Aspect	20Cr	20CrMo
Weldability	Better (lower hardenability risk)	Moderate — requires more careful preheat/PWHT
Strength–Toughness balance (core after treatment)	Good for thin/normal sections	Superior for thicker/heavily loaded sections
Cost	Lower	Higher

Choose 20Cr if: - You need a cost‑effective carburizing steel for small to medium sections where conventional case depth and core toughness are adequate. - Welding or simpler heat‑treat procedures are preferred and section sizes are moderate.

Choose 20CrMo if: - Components have large cross‑sections, deep case requirements, or very high core strength/toughness demands. - The design calls for improved temper‑resistance and reduced risk of softening in service for heavily loaded parts, and the project can accommodate slightly higher material cost and more controlled fabrication/welding procedures.

Final note: These recommendations are general. Always confirm the controlling standard, supplier chemistry, and expected heat‑treatment cycle. For critical components validate selection with mechanical testing of representative material and full weld procedure qualification when welding is required.