ST37 vs ST52 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
ST37 and ST52 are legacy European structural-steel grades that engineers, procurement managers, and manufacturing planners still encounter in specifications, material certificates, and legacy drawings. The selection dilemma typically centers on trade-offs between strength and cost, and between weldability/formability and load-bearing performance. In short: ST52 delivers significantly higher strength and design stress capacity, while ST37 offers easier fabrication and lower material cost.
These two grades are commonly compared because they occupy adjacent positions in structural-steel families: one is a lower-strength, highly workable carbon steel suitable for general construction; the other is a higher-strength structural steel intended where higher yield/tensile properties reduce section sizes or dead weight. Understanding composition, microstructure, heat-treatment response, and fabrication implications is essential for specifying the right grade for a given application.
1. Standards and Designations
- Historical/Regional standards:
- DIN 17100: original ST37, ST52 designations (older German standard).
- EN 10025: modern European equivalents (e.g., S235 series ≈ ST37; S355 series ≈ ST52 in engineering practice).
- ASTM/ASME: no direct one-to-one (ASTM A36, A572 grade 50 are approximate functional equivalents).
- JIS/GB: different systems; local equivalents vary and require cross-referencing.
- Classification by type:
- ST37: plain carbon/low-alloy structural steel (non-stainless, non-tool).
- ST52: higher-strength structural carbon/low-alloy steel (often thermomechanically or microalloyed to raise yield).
- Both are not stainless or tool steels; they are typically considered construction-grade carbon/microalloyed steels (HSLA-like behavior in modern equivalents).
2. Chemical Composition and Alloying Strategy
Below are typical composition windows used for general engineering guidance. Exact limits depend on the specific sub-grade, supplier, and applicable standard; treat the numbers as representative mass-percent ranges for common commercial variants.
| Element | Typical ST37 (representative) | Typical ST52 (representative) |
|---|---|---|
| C | ≤ ~0.17–0.20 % | ≤ ~0.20–0.24 % |
| Mn | ≤ ~1.40 % | ≤ ~1.40–1.60 % |
| Si | ≤ ~0.40 % | ≤ ~0.20–0.50 % |
| P | ≤ 0.035 % (max) | ≤ 0.035 % (max) |
| S | ≤ 0.035 % (max) | ≤ 0.035 % (max) |
| Cr | ≤ 0.30 % | trace to ≤ 0.30 % |
| Ni | trace | trace |
| Mo | trace | trace |
| V | usually none | possible microalloying (≤ 0.10 %) |
| Nb | usually none | possible microalloying (≤ 0.05 %) |
| Ti | usually none | possible trace |
| B | trace if present | trace if present |
| N | trace | trace |
Alloying strategy and effects: - ST37: composition emphasises very low carbon and minimal alloying—aimed at good weldability, ductility, and simple hot-rolled processing. - ST52: slightly higher carbon and controlled additions (or microalloying with V/Nb/Ti) and tighter control on Mn/Si produce higher yield and tensile strengths by promoting finer ferrite-pearlite microstructures and precipitation strengthening; these changes raise hardenability and strength at the cost of somewhat reduced formability/weldability compared with ST37.
3. Microstructure and Heat Treatment Response
- Typical microstructures (as-hot-rolled):
- ST37: coarse to fine ferrite and pearlite depending on cooling rate; largely ferritic matrix with low pearlite content. The microstructure is forgiving to standard fabrication and welding.
- ST52: finer-grained ferrite-pearlite or ferrite-bainite mix, especially when microalloyed or produced with thermomechanical control processes (TMCP). Precipitation of very fine carbides/niobium/titanium carbonitrides in microalloyed variants helps raise yield strength.
- Heat-treatment response:
- Normalizing/refining grain size: Both grades benefit from normalizing in critical components to refine grain size and improve toughness; ST52 typically shows greater strength gains from controlled cooling (TMCP) and normalizing.
- Quenching & tempering: Not standard practice for these grades in structural uses; turning either into quenched-and-tempered conditions is possible but would change the designation and properties—the modern approach to higher strength for structural steels is TMCP and microalloying rather than hardening-tempering cycles.
- Thermo-mechanical processing: ST52-equivalent steels are often produced by TMCP to obtain higher strength with acceptable toughness; ST37 typically receives simpler hot-rolling passes with less strengthening from controlled deformation.
4. Mechanical Properties
Representative mechanical property ranges for design guidance (actual certificate values vary with thickness, heat-treatment, and sub-grade).
| Property | ST37 (representative) | ST52 (representative) |
|---|---|---|
| Minimum Yield Strength (Rp0.2) | ~235 MPa | ~355 MPa |
| Tensile Strength (Rm) | ~360–510 MPa | ~510–680 MPa |
| Elongation (A, % on 50 mm) | ≥ ~22–26 % | ≥ ~16–22 % |
| Impact Toughness (typical testing) | 20–27 J at +20 °C (varies) | 27 J at −20 °C (J2 variants) possible |
| Hardness (HB) | ~120–160 HB | ~150–220 HB (higher due to strength) |
Interpretation: - ST52 is clearly the stronger grade (higher yield and tensile). That strength gain stems from composition control and processing (TMCP, microalloying). - ST37 is generally more ductile and easier to deform plastically; it often exhibits comparable or adequate toughness for ambient-temperature structural applications. - Toughness depends heavily on sub-grade (e.g., J0, J2 variants), thickness, and heat treatment; many ST52-equivalent steels are available with improved impact properties at sub-zero temperatures.
5. Weldability
Weldability depends on carbon content, carbon equivalents, and presence of microalloying elements that increase hardenability. Useful indices:
-
Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
Pcm (Ito-Bessyo): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - ST37 generally has a lower $CE_{IIW}$ and $P_{cm}$ than ST52 due to lower carbon and fewer hardenability-raising alloying elements; therefore it is easier to weld with lower preheat and less risk of cold cracking. - ST52's microalloying or slightly higher C and Mn increase hardenability; this raises the risk of hard martensitic microstructures in the heat-affected zone (HAZ) under rapid cooling, requiring controlled preheat, interpass temperature, and possibly post-weld heat treatment (PWHT) in thicker sections. - For critical welded structures, perform a weldability assessment based on measured composition and thickness using $CE_{IIW}$ and $P_{cm}$ thresholds, and include appropriate welding procedure specifications (PQR/WPS).
6. Corrosion and Surface Protection
- Both ST37 and ST52 are non-stainless carbon steels and are vulnerable to general and atmospheric corrosion.
- Typical protection strategies:
- Hot-dip galvanizing for outdoors/storage-exposed components.
- Organic coatings (epoxy primers, polyurethane topcoats) for enhanced aesthetics and corrosion resistance.
- Metallizing (thermal spray) or sacrificial anodes in aggressive environments.
- PREN (pitting resistance equivalent number) is not applicable to non-stainless carbon steels. For stainless or duplex steels one would use: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but this index does not apply to ST37/ST52 materials.
7. Fabrication, Machinability, and Formability
- Formability:
- ST37: better cold formability and bending performance because of lower yield strength and higher elongation; easier to roll, press-brake, and stretch-form.
- ST52: reduced formability; springback is higher and minimum bend radii are larger for the same thickness.
- Machinability:
- Higher-strength microalloyed steels (ST52) often machine slightly less easily than lower-strength steels; tool wear may increase due to higher tensile strength and hard phases.
- Both grades respond well to standard cutting fluids and tooling practices; selection of cutting speeds and feeds must account for strength difference.
- Joining and finishing:
- ST37: threading, cold punching, and forming operations are straightforward.
- ST52: may require more robust tooling and press capacities; hole-making and threading may need tougher tooling and slower feeds.
8. Typical Applications
| ST37 (typical uses) | ST52 (typical uses) |
|---|---|
| General structural members: beams, columns, purlins for non-critical loads | High-strength structural members: crane rails, heavy beams where section reduction is critical |
| Secondary structures, frames, non-critical chassis components | Welded pressure frames, heavy machinery frames, lift components |
| Pipework for low-pressure services, general fabricated parts | Applications requiring higher load capacity with reduced section thickness (e.g., truck chassis, bridge components) |
| Fabrication where cost and ease of welding/forming are priorities | Applications where weight reduction, higher safety factors, or smaller cross-section are required |
Selection rationale: - Choose ST37 when fabrication ease, cost, and adequate ambient-temperature toughness dominate. Choose ST52 when design requires higher yield strength to reduce section sizes or meet higher load/stiffness criteria.
9. Cost and Availability
- Relative cost: ST37 is usually less expensive per tonne than ST52 because of simpler chemistry and broader production routes. ST52 commands a premium for higher-strength processing and possible microalloying.
- Availability: Both grades or their modern EN equivalents (S235 / S355 families) are widely available in plate, coil, structural shapes, and sheet; availability by thickness and sub-grade (impact-tested variants) depends on mill offerings and regional supply chains.
- Procurement tip: specify chemical and mechanical acceptance criteria and required toughness temperature on purchase orders to avoid ambiguity between legacy ST names and current EN designations.
10. Summary and Recommendation
| Metric | ST37 | ST52 |
|---|---|---|
| Weldability | Excellent (lower CE) | Good to moderate (requires controls) |
| Strength–Toughness balance | Lower strength, high ductility/toughness | Higher strength, good toughness if specified and processed |
| Cost | Lower | Higher |
Conclusion and direct recommendations: - Choose ST37 if you prioritize fabrication ease, lower cost, and good weldability/formability for general structural and non-critical applications where lower yield strength is acceptable. - Choose ST52 if you need higher yield and tensile strength to reduce section size or weight, or to meet higher load or fatigue requirements—accepting higher material cost and the need for more controlled welding and forming procedures.
Practical next steps for specification: - Always cross-reference legacy ST designations to current standards (e.g., S235 / S355) and require mill test certificates showing chemistry and mechanical properties. - For welded, thick, or cold-service components, calculate carbon equivalent indices and specify welding preheat/PWHT and impact requirements as relevant. - When in doubt, run a simple trade-off analysis (mass reduction vs. fabrication cost vs. material premium) and perform weld procedure qualification where required by code or contract.