Q345R vs Q390R – Composition, Heat Treatment, Properties, and Applications

Introduction

Q345R and Q390R are low-alloy, pressure-vessel-designated steels widely used in Chinese and international fabrication for boilers, pressure vessels, and cryogenic containers. Engineers, procurement managers, and manufacturing planners commonly face a selection decision between these two grades balancing cost, manufacturability, weldability, and required mechanical performance. Typical trade-offs include choosing lower-cost, more ductile material versus a higher-strength alternative that reduces section thickness or weight.

The principal technical distinction between Q345R and Q390R is their design strength level: Q390R is specified for a higher minimum yield and is therefore used where a higher load capacity or reduced thickness is required. Because both are non-stainless pressure-vessel steels with similar alloying philosophies, they are frequently compared when optimizing designs for strength, toughness, and fabrication.

1. Standards and Designations

• GB (China): Q345R and Q390R are designated in Chinese GB/T standards for pressure vessel steels. The suffix “R” indicates suitability for welded pressure vessel plates.
• EN / ISO: Rough performance equivalents are often compared to EN S355 (for Q345 series) and higher-strength structural steels, but direct replacements must be checked against pressure-vessel standards.
• ASME / ASTM: There is no direct one-to-one ASME grade; users typically map to ASTM A516/A572 families by mechanical properties and allowable stresses.
• JIS: Japanese standards use different nomenclature; mapping requires property-by-property checks.

Classification: both Q345R and Q390R are carbon-manganese low-alloy steels (HSLA-type for pressure vessel use), not tool steels or stainless steels. They are pressure-vessel (R) variants with required impact toughness at specified temperatures.

2. Chemical Composition and Alloying Strategy

Table: general qualitative presence of common elements

Element	Q345R (typical)*	Q390R (typical)*	Functional role
C	Low–medium	Low–medium (often slightly lower)	Strength control; higher C increases strength and hardness but reduces weldability and toughness.
Mn	Medium	Medium–high	Deoxidation and solid-solution strengthening; aids hardenability.
Si	Trace–moderate	Trace–moderate	Deoxidizer; small effect on strength.
P	Trace (controlled low)	Trace (controlled low)	Impurity—kept low to maintain toughness.
S	Trace (controlled low)	Trace (controlled low)	Impurity—controlled for weldability and toughness.
Cr	Trace–low	Trace–low	May be present in small amounts for hardenability/toughness.
Ni	Trace–low	Trace–low	Improves toughness at low temperatures when present.
Mo	Trace–low	Trace–low	Hardenability and creep resistance if added.
V	Trace (microalloying possible)	Trace (microalloying more likely)	Microalloying for precipitation strengthening (grain refinement).
Nb (Nb,V combined)	Possible trace	Possible trace	Grain refinement and strength after thermo-mechanical processing.
Ti	Possible trace	Possible trace	Deoxidation and control of grain growth.
B	Not typical	Not typical	Very low levels may be used in some HSLA steels to boost hardenability (rare).
N	Controlled low	Controlled low	Can form nitrides; controlled for toughness.

*“Typical” indicates common alloying strategies for these pressure-vessel steels; consult the specific manufacturer/standard for certified element limits and batch certificates.

Alloying strategy summary: - Both grades rely principally on controlled carbon and manganese with selective microalloying (Nb, V, Ti) and trace elements for toughness and strength. - Q390R is typically produced to achieve a higher proof strength; producers may tighten carbon control, increase microalloying or apply thermomechanical processing to reach the higher level without excessive carbon content that would harm weldability or toughness.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Delivered condition for both grades is usually normalized or controlled-rolled plate with a ferrite–pearlite or fine-grained bainitic-ferritic microstructure depending on cooling rates and microalloying. - Q345R, designed for a lower strength level, often exhibits more equiaxed ferrite and pearlite with larger ferrite regions, which contributes to ductility. - Q390R aims for a finer-grained microstructure with stronger matrix contributions (more tempered bainite or refined ferrite) achieved by tighter rolling schedules or microalloy precipitation, giving higher yield strength.

Heat treatment response: - Normalizing: Both grades respond to normalizing by refining grain size and improving uniformity; normalizing is often specified for pressure-vessel plates to assure toughness. - Quenching & tempering: Not typical as a standard delivery for these "as-rolled" pressure-vessel plates, but possible for bespoke higher-strength variants; Q390R can be engineered to benefit more from quench-and-temper routes if a manufacturer requires higher strength with retained toughness. - Thermo-mechanical controlled processing (TMCP): Widely used to produce Q390R to reach higher strength while maintaining acceptable toughness and weldability—microalloying elements precipitate and refine austenite grain size during rolling.

4. Mechanical Properties

Table: comparative mechanical attributes (qualitative with specified minimum indicated where inherent)

Property	Q345R	Q390R	Comment
Specified minimum yield strength	~345 MPa (by designation)	~390 MPa (by designation)	Core distinction—Q390R has higher statutory yield.
Tensile strength	Moderate	Higher	Q390R generally achieves higher ultimate tensile strength.
Elongation (ductility)	Higher ductility	Slightly lower ductility	Higher-strength steels usually show reduced uniform elongation.
Impact toughness	Good (specified at design temp)	Good but may require stricter control	Both graded for impact, but Q390R requires process control to retain toughness at higher strength.
Hardness	Moderate	Higher	Higher strength correlates with increased hardness.

Explanation: - Q390R delivers a higher strength-to-weight ratio and enables thinner sections for the same loading. The trade-off is that achieving higher strength can reduce ductility and makes microstructure control more critical for preserving low-temperature toughness. - For many pressure-vessel applications where toughness is specified at particular temperatures, both grades are engineered to meet impact energy requirements; suppliers verify via charpy V-notch testing at the specified temperature.

5. Weldability

Weldability drivers: - Carbon content and combined hardenability determine susceptibility to cold cracking and need for preheat/post-heat. - Microalloying (Nb, V) can increase hardenability locally at the weld heat-affected zone (HAZ).

Useful indices (qualitative use; do not substitute for procedure qualification): - Carbon equivalent, IIW form: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - 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: - Lower $CE_{IIW}$ and $P_{cm}$ values imply easier weldability with standard consumables and lower preheat; as carbon and certain alloying elements increase, so does the need for preheat, controlled interpass temperatures, and post-weld heat treatment. - Q345R, with its lower design strength and often slightly higher allowable carbon ranges, typically shows better inherent weldability than Q390R when all other factors are equal. However, modern Q390R produced with low carbon and TMCP can have acceptable $CE_{IIW}/P_{cm}$ values comparable to Q345R. - Welding procedure qualification (WPS/PQR) and hydrogen control are essential for both grades, especially for thick sections and low-temperature service.

6. Corrosion and Surface Protection

• Neither Q345R nor Q390R is stainless or corrosion-resistant by composition. Corrosion protection is achieved by:
• Coatings: solvent-borne or water-borne paints, epoxy primers, and polyurethane topcoats.
• Metallic protection: hot-dip galvanizing or metallizing (zinc/aluminum) for atmospheric service.
• Corrosion allowances: specifying thicker plate to account for predictable corrosion rates.
• PREN (pitting resistance equivalent number) is not applicable to these non-stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use PREN only for stainless alloys; for Q345R/Q390R consider cathodic protection, barrier coatings, and appropriate material selection for aggressive environments.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades are readily flame-cut, plasma-cut, and laser-cut. Higher strength of Q390R may require slower cutting speeds to control edge quality in thick sections.
• Bending/forming: Q345R, with lower yield, is generally easier to bend to tight radii and to cold-form without springback or cracking. Q390R, being stronger, exhibits higher springback and a reduced bendability window—form design should be reviewed and drawing tolerances adjusted.
• Machinability: Both are machinable with standard carbon steel practices. Higher strength and higher hardness of Q390R can slightly reduce tool life; select cutting tool materials and speeds accordingly.
• Surface finishing: Grinding and shot-blasting proceed similarly; heat input during welding or heavy machining can locally alter properties and should be considered in fabrication sequences.

8. Typical Applications

Q345R – Typical Uses	Q390R – Typical Uses
Low- to moderate-pressure boilers and pressure vessels where weight optimization is not critical; structural sections and welded fabrications requiring good ductility and toughness	Pressure vessels, storage tanks, and structures where reduced wall thickness or weight savings are prioritized; heavier duty process vessels and components under higher static loads
Piping and components with demanding low-temperature toughness needs where a bit more ductility is desired	Fabrications where allowable stress or design codes permit benefit from higher yield (e.g., to reduce plate thickness)
General-purpose fabricated tanks and equipment where welding ease and cost are primary concerns	Applications where mechanical strength is critical while maintaining welded integrity—given tighter process controls

Selection rationale: - Select Q345R when fabrication ease, formability, and minimal welding constraints outweigh the need to minimize weight. - Select Q390R when structural optimization, weight savings, or higher design pressures require the higher yield strength—provided quality controls ensure toughness.

9. Cost and Availability

• Relative cost: Q390R typically commands a premium over Q345R because of tighter processing controls, possible microalloying, and certification for higher strength. The premium varies by market and batch.
• Availability: Both grades are commonly produced, but availability depends on plate thickness, width, and required impact-temperature qualification. Q345R may be more broadly available in a wider range of sizes; Q390R availability can be more variable for thicker plates or very low-temperature-certified batches.
• Procurement: Specify required mechanical properties, impact temperature, and weld-related consumable compatibility. Request mill test certificates (MTC) and heat numbers to ensure traceability.

10. Summary and Recommendation

Summary table (qualitative)

Attribute	Q345R	Q390R
Weldability	Good	Good–Moderate (needs process control)
Strength–Toughness balance	Moderate strength with good ductility/toughness	Higher strength; toughness achievable with tighter control
Cost	Lower	Higher

Recommendations: - Choose Q345R if: - Fabrication simplicity, higher ductility, and lower material cost are priorities. - Design does not require the higher yield strength and thickness reduction. - Ease of welding and forming is more important than minimizing weight.

• Choose Q390R if:
• Higher design yield strength is required to reduce wall thickness or weight.
• Project economics favor thinner sections despite a higher unit material cost.
• The manufacturer can supply certified material meeting toughness requirements and controlled processing (TMCP, low-C, microalloying) to manage weldability and HAZ properties.

Final note: Always verify the exact chemical and mechanical limits from the supplier’s mill certificate and qualify welding procedures for the specific plate thickness and service temperature. For pressure-vessel work, comply with the applicable code or standard and document impact-testing regimes and nondestructive examination requirements when selecting between Q345R and Q390R.