15CrMo vs 20CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

15CrMo and 20CrMo are two chromium–molybdenum alloy steels commonly encountered in pressure-vessel, power-generation, and mechanical component applications. Engineers and procurement teams often decide between them when balancing requirements such as elevated-temperature strength and creep resistance, vs. hardness and through-thickness strength for heavily loaded parts. Typical decision contexts include whether the part will operate for long durations at elevated temperature (favoring lower carbon/higher temper stability) or whether higher as-quenched strength and hardenability are needed (favoring the higher-carbon grade).

The primary technical distinction between these grades lies in their carbon content and the resulting effect on hardenability and tempering behavior: the lower-carbon grade offers better toughness and serviceability at elevated temperatures, while the higher-carbon grade can achieve higher strength and wear resistance after appropriate heat treatment. Because both are Cr–Mo steels, they are compared often for mid-temperature pressure-service components and for structural/mechanical parts where weldability, heat treatment response, and cost all matter.

1. Standards and Designations

• Common standards and cross-references:
• GB/T (China): designations such as 15CrMo and 20CrMo appear in GB specifications for pressure-vessel steels.
• EN / DIN: similar Cr–Mo steels are found under EN/DIN families (e.g., 13CrMo4-5; exact equivalence depends on chemistry and heat-treatment).
• JIS (Japan) and ASTM/ASME (USA): equivalent or similar-purpose steels exist, but exact grade matches require chemical and mechanical confirmation.
• Classification:
• Both 15CrMo and 20CrMo are alloy steels (Cr–Mo low-alloy steels), not stainless, not tool steels, and not HSLA in the strict sense. They are often used for pressure vessels, piping, and mechanical parts exposed to elevated temperature.

2. Chemical Composition and Alloying Strategy

Table: Typical nominal composition ranges (representative; verify against the specific standard or mill certificate for final design).

Element	15CrMo (typical ranges)	20CrMo (typical ranges)
C	0.10–0.18 wt%	0.17–0.24 wt%
Mn	0.35–0.65 wt%	0.35–0.65 wt%
Si	0.10–0.37 wt%	0.10–0.37 wt%
P	≤ 0.035 wt%	≤ 0.035 wt%
S	≤ 0.035 wt%	≤ 0.035 wt%
Cr	~0.8–1.1 wt%	~0.8–1.3 wt%
Mo	~0.12–0.25 wt%	~0.12–0.30 wt%
Ni	≤ 0.30 wt% (trace)	≤ 0.30 wt% (trace)
V, Nb, Ti, B, N	Not typically added in significant amounts; may be present in trace/microalloying levels	Same

Notes: - These ranges are illustrative of commonly encountered mill chemistry for the two grade names; specific standards (GB/T, EN, JIS, ASTM) and heat numbers determine the exact limits. - Alloying strategy: Cr and Mo increase hardenability, strength at temperature, and tempering resistance. Carbon raises as-quenched strength and hardenability but reduces ductility/toughness and weldability when increased. Manganese and silicon are deoxidizers and contribute to strength and hardenability.

3. Microstructure and Heat Treatment Response

• As-rolled / normalized microstructure:
• 15CrMo in normalized condition is typically a tempered ferrite–pearlite or fine bainitic microstructure with relatively low retained hardness and good toughness; chosen for pressure parts operating at elevated temperature.
• 20CrMo, with higher carbon and comparable Cr–Mo, can form finer pearlite or transform to bainite/martensite more readily during fast cooling, giving increased hardness and strength after quench and temper.
• Heat treatment effects:
• Normalizing/refining: both steels respond to normalizing (air cooling from the austenitizing temperature) by producing fine ferrite–pearlite or bainite depending on cooling rate; 20CrMo tends to develop higher hardness due to carbon content.
• Quench and temper: 20CrMo achieves higher as-quenched strength and higher temper-hardened strength but is more susceptible to quench cracking and requires stricter control of preheat and interpass temperatures for welding. 15CrMo achieves adequate strength for pressure-vessel service with reduced quench sensitivity.
• Thermo-mechanical processing: controlled rolling and accelerated cooling can improve strength and toughness for both grades, but the lower-carbon grade generally gives a more damage-tolerant microstructure for elevated-temperature service.

4. Mechanical Properties

Table: Comparative properties (qualitative/typical behavior; check product certification for exact values)

Property	15CrMo	20CrMo	Notes
Tensile Strength	Moderate	Higher	20CrMo attains higher tensile after quench & temper due to higher C/hardenability
Yield Strength	Moderate	Higher	20CrMo yields at higher stress after heat treatment
Elongation (%)	Higher (more ductile)	Lower (less ductile)	Higher carbon reduces ductility
Impact Toughness	Better at elevated temperature	Good at ambient when quenched/tempered but lower at high T	15CrMo intended for sustained elevated-temperature toughness
Hardness (HRC/HRB)	Lower (easier machining/forming)	Higher (when heat treated)	20CrMo can reach higher hardness after appropriate heat treatment

Interpretation: - For comparable heat-treatment conditions aimed at pressure-vessel service (tempered conditions), 15CrMo typically offers a more ductile and tough response at service temperatures, while 20CrMo can be used where greater as-quenched strength and wear resistance are required. - Designers must match heat treatment to the service environment: for creep-resistance, tempering stability and lower carbon can be desirable; for load-bearing components requiring higher yield/tensile, higher carbon/hardenability may be prioritized.

5. Weldability

Weldability depends primarily on carbon equivalent and alloying. Two widely used empirical 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: - 20CrMo, having greater carbon content, will generally have a higher $CE_{IIW}$ and $P_{cm}$ than 15CrMo for the same levels of Mn, Cr, and Mo — indicating greater susceptibility to HAZ hardening and cold cracking unless appropriate preheat and post-weld heat treatment (PWHT) are applied. - 15CrMo’s lower carbon reduces the need for heavy preheat and allows more forgiving welding practices, though PWHT is still often required for pressure-vessel service to relieve residual stresses and temper the HAZ. - Both grades contain Cr and Mo which increase hardenability; welding procedures (preheat, interpass, and PWHT) must be qualified per code (e.g., ASME Section IX) for pressure applications.

6. Corrosion and Surface Protection

• Neither 15CrMo nor 20CrMo is stainless; both require surface protection in corrosive environments.
• Typical protections: painting, industrial coating systems, galvanizing (where appropriate for design temperature and service), or cladding with corrosion-resistant alloys for more aggressive environments.
• PREN is not applicable for these non-stainless low-alloy steels, but when discussing corrosion resistance for stainless alloys one would use:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• For high-temperature oxidation/scale resistance, Cr and Mo content helps, but for true corrosion resistance (chloride, acidic media) stainless alloys or surface cladding are required.

7. Fabrication, Machinability, and Formability

• Machinability: 15CrMo (lower carbon) is generally easier to machine than 20CrMo in similar heat-treated conditions. Higher hardness in 20CrMo raises cutting forces and tool wear.
• Forming/bending: 15CrMo better tolerates cold forming and bending due to higher ductility; 20CrMo may require lower bend radii or annealing/normalizing before forming.
• Finishing: Surface grinding, polishing, and shot-blasting are similar, but 20CrMo’s higher hardness may necessitate more aggressive tooling or slower feed rates.
• Welding and fabrication: both grades typically require preheat and PWHT when used in pressure applications; the degree and temperature depend on carbon equivalent and thickness.

8. Typical Applications

Table: Typical uses

15CrMo	20CrMo
Boiler and pressure-vessel components for moderate elevated temperatures	Mechanical shafts, studs, bolts, and load-bearing components requiring higher quenched/tempered strength
Piping and fittings in power plants where toughness at temperature is required	Gears, heavy-duty couplings, and structural parts where higher strength or wear resistance is needed after heat treatment
Heat-exchanger tubes, headers, and flanges where creep resistance and temper stability are important	Press-fit or shrunk-fit components and parts subjected to cyclic mechanical loads (after suitable heat treatment)

Selection rationale: - Choose 15CrMo where sustained temperature performance, ductility, and weldability with lower hydrogen-cracking risk are priorities (pressure-vessel and piping). - Choose 20CrMo where higher strength, hardness, and wear resistance are required and where controlled heat treatment is feasible.

9. Cost and Availability

• Raw material cost: both are Cr–Mo alloy steels; material cost differences are modest and largely driven by local supply, form (plate, bar, tube), and processing requirements.
• Processing cost: 20CrMo can carry higher processing costs due to stricter heat-treatment/weld controls and potentially more expensive machining/tooling if higher hardness is targeted.
• Availability: Both grades are widely available in common product forms (plate, bar, seamless tube) in many industrial markets; specific heat-treated conditions and large-diameter pressure-material certifications may be more limited and require longer lead times.

10. Summary and Recommendation

Table: High-level comparison

Attribute	15CrMo	20CrMo
Weldability	Better (lower CE)	Requires stricter preheat/PWHT (higher CE)
Strength–Toughness balance	Good toughness, moderate strength	Higher achievable strength, lower ductility when hardened
Cost (material + processing)	Lower-to-moderate	Moderate-to-higher (depending on heat treatment)

Recommendations: - Choose 15CrMo if you need a Cr–Mo alloy for moderate elevated-temperature service, pressure-vessel components, or piping where long-term temper stability, toughness, and more forgiving weldability are important. - Choose 20CrMo if you need higher through-thickness strength or hardenability for mechanical parts, gears, shafts, or components that will be quenched and tempered to a higher hardness and strength level, and where you can control heat treatment and welding procedures.

Final note: Always confirm the required mechanical properties, heat-treatment condition, and mill certificate chemistry against the governing code or specification for your application. For welded pressure equipment, follow the applicable design code (ASME, EN, GB/T) and validate welding procedures and PWHT requirements based on the calculated carbon equivalent and thickness.