Q345R vs Q370R – Composition, Heat Treatment, Properties, and Applications

Introduction

Q345R and Q370R are Chinese-designated pressure-vessel steels commonly considered when specifying plate and shell materials for boilers, pressure vessels, and similar welded structures. Engineers and procurement professionals often weigh trade-offs such as weldability versus higher yield strength, toughness at low temperature versus fabrication cost, and availability in plate thicknesses or normalized condition.

The principal distinction between the two is that Q370R represents a higher-strength, updated pressure-vessel grade that uses refined alloying and processing to raise yield/tensile performance while aiming to preserve toughness and weldability. Because both appear in similar application spaces, designers routinely compare them to decide whether higher structural performance justifies potential impacts on welding procedure, heat treatment, and cost.

1. Standards and Designations

• GB (China): Q345R and Q370R are identified in Chinese national standards for pressure vessel steels and related normative documents. Specific chemical and mechanical limits are defined in GB/T and corresponding pressure-vessel codes.
• ASME / ASTM: There is no direct one-to-one mapping; analogous Western grades for pressure applications include ASTM A516 (for carbon steel) and various normalized plates, but comparisons should be made on specific chemistry and mechanical results rather than name equivalence.
• EN (Europe) / JIS (Japan): European and Japanese standards specify their own pressure-vessel steels (for example, EN 10028 series for flat products), and selection should be cross-referenced by required properties and testing rather than nominal grade names.

Classification: Both Q345R and Q370R are low-alloy carbon steels intended for pressure-vessel service (non-stainless). They fall into the broad HSLA/pressure-vessel plate category, with Q370R typically having a stronger alloying/thermo-mechanical approach to reach higher yield levels.

2. Chemical Composition and Alloying Strategy

The two grades share the same principal elements (C, Mn, Si, P, S) but differ in permitted concentrations and in microalloying additions that influence hardenability, precipitation strengthening, and grain refinement.

Table: Typical compositional features (representative ranges; verify supplier certificates for exact values)

Element	Q345R — Typical range / role	Q370R — Typical range / role
C	Low carbon, controlled for welded pressure-vessel performance (e.g., ~≤0.20%)	Slightly higher or similar C control to reach higher yield; still kept low enough for weldability
Mn	Moderate (deoxidation, strength)	Moderate to slightly higher to assist strength and hardenability
Si	Small deoxidizer (≤~0.35%)	Similar to Q345R
P	Tight control (impurity limit)	Similar or slightly tighter control in some specs
S	Low sulfur for toughness	Low sulfur; controlled similarly
Cr, Ni, Mo	Generally minimal in Q345R; occasional small additions in special variants	May include small amounts (trace to low tenths of percent) in some manufacturers to boost hardenability and strength
V, Nb, Ti	Usually absent or at very low levels in basic Q345R	Q370R variants more likely to include microalloying (V, Nb, Ti) for precipitation strengthening and grain refinement
B, N	Not a primary alloying strategy; N controlled	N controlled; trace B occasionally used in microalloyed steels for hardenability control

Notes: - Exact chemical limits are specified by the supplier and applicable standard. The table is intended to highlight alloying strategies: Q345R uses conservative chemistry to balance strength and weldability; Q370R typically relies on slightly higher alloying and/or microalloying plus processing control to achieve higher yield numbers while retaining toughness.

How alloying affects properties: - Carbon increases strength and hardenability but reduces weldability and toughness if excessive. - Manganese increases strength and hardenability and helps deoxidation. - Microalloying with Nb, V, Ti enables strength through fine precipitates and grain refinement, improving strength without proportionally increasing carbon. - Small additions of Cr, Mo, Ni increase hardenability and elevated-temperature strength but are used sparingly in pressure-vessel steels to control costs and maintain weldability.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Q345R: Produced to yield a fine-grained ferrite–pearlite or acicular ferrite microstructure after controlled rolling and normalization; the microstructure is chosen to provide a balance of toughness and ductility at common service temperatures. - Q370R: Achieves higher yield by a combination of slightly higher dislocation/solute strengthening and microalloy precipitates. Microstructure often comprises finer ferrite with controlled pearlite and enhanced bainitic fraction in some thermo-mechanical routes.

Response to heat treatment and processing: - Normalizing: Both grades respond to normalizing with grain refinement and predictable mechanical properties. Normalizing temperatures must be controlled to prevent over-tempering of microalloy precipitates in Q370R. - Quenching & tempering: Not standard for these pressure-vessel plates (usually produced as normalized or controlled-rolled), but if applied, Q370R’s alloying will affect hardenability and tempering response more strongly than Q345R. - Thermo-mechanical controlled processing (TMCP): Particularly important for Q370R to achieve higher strength with required toughness; TMCP helps produce fine-grained microstructures and uniform properties through heavy rolling plus accelerated cooling.

4. Mechanical Properties

Table: Typical mechanical-property comparison (representative ranges; confirm with certificate)

Property	Q345R — Typical	Q370R — Typical
Yield strength (MPa)	Nominal ~345	Nominal ~370
Tensile strength (MPa)	Typical range ~470–630	Typical range ~500–700
Elongation (%)	Typically ≥20 (depending on thickness)	Typically ≥17–20 (slightly lower at equal thickness)
Impact toughness	Specified impact energy at temperature (designed for acceptable low-temp toughness)	Targeted to match Q345R at similar test temps but sometimes requiring stricter control
Hardness (HB or HRC)	Moderate (consistent with ductile plate)	Slightly higher due to added strength mechanisms

Interpretation: - Q370R is stronger (higher nominal yield and higher tensile capability) due to alloying and processing. That higher strength can come with a modest reduction in uniform elongation and potentially increased sensitivity to welding thermal cycles if higher hardenability elements are present. - Toughness is a design criterion for both grades; Q370R manufacturers typically control composition and process to preserve impact energy requirements required by pressure-vessel codes.

5. Weldability

Weldability considerations hinge on carbon equivalent and hardenability. Two useful indices:

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

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

Qualitative interpretation: - A higher $CE_{IIW}$ or $P_{cm}$ indicates greater hardenability and higher risk for martensite formation in the heat-affected zone (HAZ), increasing preheat/post-weld heat treatment (PWHT) requirements. - Q345R typically has a lower carbon equivalent and thus easier weldability with conventional welding procedures and lower preheat demands. - Q370R, due to small increases in alloy content and microalloying, can have a higher carbon equivalent; this requires more careful welding procedure specification (preheat, interpass temperature, filler selection, and sometimes PWHT) to avoid HAZ embrittlement or cold cracking. - Microalloying elements (Nb, V, Ti) refine grain size and can improve toughness, but they also increase high-temperature strength and can slightly raise carbon-equivalent measures; weld procedure qualification is recommended.

6. Corrosion and Surface Protection

• Both Q345R and Q370R are non-stainless low-alloy steels. They are not corrosion-resistant by chemistry and typically require surface protection for atmospheric or corrosive service.
• Common protections: hot-dip galvanizing (where appropriate for thickness and service), organic coatings (paints, epoxy, polyurethane), metallizing (sprayed zinc/aluminum), or cladding with corrosion-resistant alloys when necessary.
• The PREN formula is not applicable (non-stainless steels). For stainless materials the PREN index is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• When specifying either Q345R or Q370R for environments with corrosion risk, selection must consider the appropriate coating system and inspection regime; galvanizing may change welding and post-fabrication procedures.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades cut similarly using oxy-fuel, plasma, or laser; Q370R’s higher strength can produce slightly increased tool wear for machining adjacent fittings.
• Forming/bending: Q345R, being more ductile, is generally easier to form on the same bend radii; Q370R requires slightly larger minimum bend radii or controlled bending to avoid cracking, especially in thicker plates.
• Machinability: Increased strength and microalloying in Q370R can reduce machinability and may require adjusted feeds/speeds and tooling selection compared to Q345R.
• Heat treatment and stress relief: PWHT may be mandated by code or welding procedure more often for higher-strength grades; coordinate fabrication sequences accordingly.

8. Typical Applications

Q345R — Typical uses	Q370R — Typical uses
Low- to moderate-pressure boilers, storage vessels where ductility and weldability are prioritized	Pressure vessels and shells where higher design stress or reduced plate thickness is desirable
General structural components in fabrication of welded tanks and low-temperature vessels	Applications requiring higher allowable stresses, reduced weight, or thicker sections with maintained toughness
Large, readily welded plates where cost is primary constraint	Situations where code permits higher-strength plates to reduce weight or material thickness; retrofits that require higher capacity without geometry change

Selection rationale: - Choose Q345R when fabrication simplicity, proven weldability, and cost control are priorities. - Choose Q370R when structure-level weight savings, higher allowable stress, or higher design margins are required and when the project can accommodate stricter welding/fabrication controls.

9. Cost and Availability

• Cost: Q370R is generally more expensive per ton than Q345R because of tighter processing control, possible microalloying additions, and additional testing or certification to meet tougher mechanical targets.
• Availability: Q345R is historically more common and broadly stocked in many markets and thicknesses. Q370R availability is growing, particularly where codes recognize higher-strength pressure-vessel steels; however, product form (plate thickness, certifications) should be confirmed with mills and distributors early in procurement.

10. Summary and Recommendation

Table: Quick comparison

Attribute	Q345R	Q370R
Weldability	Very good (lower CE)	Good, but needs qualified WPS and possible PWHT
Strength–Toughness balance	Balanced for typical service	Higher strength for same or close toughness with tighter control
Cost	Lower	Higher

Recommendations: - Choose Q345R if weldability, broad availability, lower material cost, and ease of fabrication are the top priorities — for example, standard boilers, tanks, and many welded pressure parts where standard allowable stresses suffice. - Choose Q370R if you need higher yield/tensile strength to reduce plate thickness, meet a higher design stress, or optimize weight while maintaining impact resistance — provided you can implement qualified welding procedures, controlled fabrication, and accept slightly higher material and processing cost.

Final note: Always verify the manufacturer’s mill test report and the applicable code requirements (pressure-vessel standard, thickness limits, required impact temperatures) before final selection. Welding procedure qualification and material traceability are essential for both grades, and they become more critical as strength and hardenability increase.