Q345C vs Q345D – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Q345 is a widely used Chinese-designated high-strength, low-alloy structural steel family. Engineers, procurement managers, and manufacturing planners frequently confront a common selection dilemma when specifying Q345 subgrades: the design load and service environment often permit several subgrades that have nearly identical strength and fabrication characteristics but different low-temperature toughness requirements and associated qualification costs. Choosing between two adjacent subgrades such as Q345C and Q345D typically comes down to balancing cost and availability against the need for assured impact performance at lower temperatures.
The primary practical distinction between Q345C and Q345D is their required verified toughness in cold service: Q345D is specified and certified for lower-temperature impact performance than Q345C. Because their nominal chemical compositions and static strength levels are essentially the same, designers compare them chiefly on low-temperature ductility/toughness and any process implications that follow (preheat, welding procedure qualification, and fabrication controls).
1. Standards and Designations
- Chinese standard: GB/T 1591 — “Low alloy high strength structural steel” (Q345 series).
- Other regional parallels: EN S355 (structural), ASTM A572 Grade 50 (approximate equivalents in performance, not a direct chemical match).
- Classification: both Q345C and Q345D are HSLA (high-strength low-alloy) structural carbon steels with microalloying additions used to achieve strength and toughness.
Note: Q345 subgrades (A, B, C, D, E) are distinguished primarily by mandatory impact test temperature and energy acceptance; they are not separate alloy families like stainless vs. carbon.
2. Chemical Composition and Alloying Strategy
| Element | Typical role in Q345 series | Typical composition range (representative) |
|---|---|---|
| C (Carbon) | Strength control, hardness, weldability influence | ~0.12–0.20 wt% (low-carbon design) |
| Mn (Manganese) | Deoxidation, tensile strength, hardenability | ~1.0–1.7 wt% (chief alloying element) |
| Si (Silicon) | Deoxidizer, small strength contribution | ≤ ~0.5 wt% |
| P (Phosphorus) | Impurity; embrittlement risk if high | ≤ ~0.035 wt% |
| S (Sulfur) | Impurity; affects machinability and toughness | ≤ ~0.035 wt% |
| Cr (Chromium) | Hardenability and corrosion resistance (minor) | ≤ ~0.3 wt% (trace in many melts) |
| Ni (Nickel) | Toughness at low temperatures (if present) | ≤ ~0.3 wt% (usually low) |
| Mo (Molybdenum) | Hardenability and strength (minor) | Typically ≤ ~0.08 wt% |
| V, Nb, Ti (Microalloying) | Grain refinement, precipitation strengthening | Trace additions (ppm to ~0.02 wt%) |
| B (Boron) | Hardenability in very small amounts (rare) | Trace if present |
| N (Nitrogen) | Affects precipitates and toughness | Controlled at low levels |
Comments: - Q345 grades are designed as low-carbon, Mn-alloyed steels. Microalloying (V, Nb, Ti) may be used by mills to achieve the 345 MPa yield specification with controlled grain size and precipitation strengthening rather than by raising carbon. - The practical result is that Q345C and Q345D typically share nearly identical chemical compositions; the subgrade designation reflects inspection and impact testing at different temperatures rather than fundamentally different alloy strategies.
3. Microstructure and Heat Treatment Response
- Typical microstructure: ferrite–pearlite (or ferrite with fine bainitic constituents depending on rolling and cooling), with microalloy precipitates if V/Nb/Ti are used. Grain refinement from microalloying increases toughness without large increases in carbon.
- Normalizing: common production route for plate and structural sections — produces a tempered ferrite–pearlite matrix with improved uniformity and toughness.
- Quenching & tempering: not typical for standard Q345 structural products; these processes are more common for higher-strength quenched steels or parts requiring specific hardness.
- Thermo-mechanical controlled processing (TMCP): widely used to achieve Q345 properties — controlled rolling and accelerated cooling refine grain size and increase strength without high carbon.
- Effect on grades: because the intrinsic chemistry is similar for Q345C and Q345D, microstructural differences arise from rolling/thermal history and mill control. To meet the lower temperature impact acceptance of Q345D, mills will control processing and heat treatment more tightly (finer grain, optimized precipitation) and perform the required impact testing.
4. Mechanical Properties
| Property | Q345C (typical) | Q345D (typical) |
|---|---|---|
| Specified minimum yield strength | 345 MPa (nominal) | 345 MPa (nominal) |
| Tensile strength | ~470–630 MPa (depends on thickness/product form) | ~470–630 MPa (similar) |
| Elongation (A%) | ~20% (varies with thickness) | ~20% (similar) |
| Impact toughness requirement | Verified by CVN at a specified temperature (moderate subzero) | Verified by CVN at a lower specified temperature (colder service) |
| Hardness | Typical structural-steel range; not a primary spec | Comparable to Q345C |
Explanation: - Strength: both grades are specified to the same minimum yield strength (345 MPa), so neither is inherently “stronger” in static load capacity. - Toughness: Q345D is qualified for colder service via impact testing at a lower temperature (stricter acceptance) and therefore provides higher assured toughness at lower temperatures. This is achieved by processing and tighter mill controls rather than grossly different chemistry. - Ductility: nominal elongation values are similar; low-temperature ductility that affects fracture behavior is the key differentiator and is validated by impact testing.
5. Weldability
Weldability of Q345 steels is generally good because of the low carbon content and controlled alloying. However, microalloying and higher Mn/hardenability can increase the need for preheat or controlled heat input in thick sections.
Useful weldability indices: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Both Q345C and Q345D have low nominal carbon and moderate Mn, giving relatively low $CE_{IIW}$ and $P_{cm}$ indices compared with high-alloy steels. This generally indicates good weldability. - Q345D’s lower-temperature toughness requirement can mean that welding procedures must be validated for the intended service temperature (preheat, interpass temperature, and post-weld heat treatment considerations). For thicker sections or complex welded structures, procedure qualification should include impact testing (or justification) relevant to the lowest service temperature. - Microalloying elements (V, Nb) and higher Mn increase hardenability locally; ensure appropriate welding parameters to avoid cold crack susceptibility in weld heat-affected zones (HAZ).
6. Corrosion and Surface Protection
- Q345C and Q345D are non-stainless carbon/alloy steels; intrinsic corrosion resistance is limited.
- Typical protection strategies: hot-dip galvanizing, zinc-rich coatings, paint systems (epoxy primers, polyurethane topcoats), or specialized corrosion-resistant coatings for marine/offshore service.
- PREN (pitting resistance equivalent number) is not applicable to Q345 steels because PREN is used for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For applications requiring significant corrosion resistance (sea water, chloride-rich atmospheres), use stainless grades or corrosion-resistant alloys rather than relying on Q345 with coatings.
7. Fabrication, Machinability, and Formability
- Machinability: Q345 steels machine similarly to other low-alloy structural steels. Lower carbon and controlled sulfur help avoid excessive tool wear; machinability depends on melt practice and microalloying.
- Formability and bending: both grades are readily formed, rolled, and cold-bent within limits determined by thickness and bend radius. Q345D may demand more attention when forming components for very low-temperature service because forming operations (and any induced cold work) can affect localized toughness; post-form heat treatment or qualification may be required for critical parts.
- Surface preparation and straightening: normal practices apply. Avoid localized overheating during flame cutting in thicker sections to prevent HAZ embrittlement; if service is cold, plan for HAZ impact verification if necessary.
8. Typical Applications
| Q345C — Typical uses | Q345D — Typical uses |
|---|---|
| General structural components (building frames, bridges where moderate cold tolerance is needed) | Structural components in lower-temperature environments (cold-climate bridges, refrigerated structures) |
| Machinery frames, cranes, general fabrication | Offshore or near-shore structures where lower-temperature toughness is required by client/spec |
| Plate, beams, channels for civil engineering | Marine topside components and deck structures with colder ambient or splash zones (with appropriate corrosion protection) |
| Welded assemblies, containers for non-cryogenic service | Applications where additional qualification/testing for low temperatures is required |
Selection rationale: - Choose Q345C where design temperatures do not approach the lower threshold that would trigger Q345D qualification, to save on testing and possibly cost. - Choose Q345D where guaranteed impact toughness at a lower temperature is required by code, client, or environmental exposure.
9. Cost and Availability
- Base material cost: Q345C and Q345D are produced from the same production route and raw materials; intrinsic material cost is similar.
- Additional cost drivers for Q345D:
- Additional mill controls and processing to meet lower-temperature toughness.
- Extra impact testing and certification at the lower temperature.
- Possible premium for stocked plate and sections certified to Q345D.
- Availability: both grades are common in plate, sheet, and structural sections in major market regions where GB/T-based steels are stocked. Q345C is often more commonly stocked; Q345D can be available on request or with lead time for certified deliveries.
10. Summary and Recommendation
| Characteristic | Q345C | Q345D |
|---|---|---|
| Weldability | Good (low C, moderate Mn); standard WPS applies | Good but with attention to lower-temp HAZ toughness and procedure qualification |
| Strength–Toughness balance | 345 MPa yield; adequate room-temp toughness | 345 MPa yield; higher assured low-temperature toughness (by test) |
| Cost & availability | Slightly more common/cheaper due to less stringent testing | Small premium and sometimes longer lead time for certification |
Recommendation: - Choose Q345C if: your design’s lowest service temperature is above the qualification temperature for Q345C, you want to minimize testing/certification costs, and you do not require verified impact toughness at lower subzero conditions. - Choose Q345D if: the structure will operate in colder climates or there is a regulatory/client requirement for impact toughness at a lower temperature; when fracture toughness at lower temperatures is a critical safety concern; or when codes require the lower test temperature for welded or thick sections.
Final note: Because the chemical bases and static strength are effectively the same for both subgrades, the selection should be driven by validated fracture-toughness requirements for the anticipated service temperature, weld procedure qualification needs, and any life-safety or regulatory constraints. Consult the project code and the mill’s mill-test-certificates (MTCs) to confirm the specific impact test temperature and acceptance criteria for the ordered product.