ST37 vs ST52 – Composition, Heat Treatment, Properties, and Applications

Introduction

ST37 and ST52 are legacy German structural-steel grades widely referenced in European practice and in many industrial supply chains. Engineers, procurement managers, and manufacturing planners commonly face the choice between these two when balancing cost, formability, weldability, and required mechanical performance for structural and fabricated components. Typical decision contexts include whether to prioritize lower cost and high ductility for general construction (favoring lower-strength steels) or to reduce section sizes and weight by specifying higher-strength steels (favoring higher-strength grades).

The principal practical difference between ST37 and ST52 is their target strength level and the corresponding alloying/microalloying strategies used to achieve that strength: ST52 is produced and processed to achieve higher yield and tensile strength than ST37, which in turn affects toughness, ductility, weldability, and forming behavior. Because both are non-stainless structural steels, they are compared frequently for applications such as beams, plates, pipelines, and welded fabrications.

1. Standards and Designations

• DIN (historical): ST37 (often written St37-2) and ST52 (St52-3) originate from the older DIN 17100 series of structural steels.
• EN equivalents: ST37 is broadly comparable to EN 10025 grade S235 (e.g., S235JR/S235J0), while ST52 occupies a higher strength class and is often considered comparable to higher-strength structural steels (some overlaps exist with EN S355 or other higher-strength steels depending on subgrade and processing).
• Other standards: ASTM/ASME use different nomenclature (e.g., A36 ~ S235 in some contexts), JIS and GB (Chinese) use their own grade systems; direct one-to-one mappings require checking property criteria and chemistry.
• Classification: Both ST37 and ST52 are plain carbon/low-alloy structural steels (non-stainless). ST37 behaves as a conventional low-carbon structural steel; ST52 often includes microalloying elements (Nb, V, Ti) or controlled thermomechanical processing to reach higher strength without heavy quench-and-temper alloying.

2. Chemical Composition and Alloying Strategy

The chemistry of ST37 and ST52 reflects the performance target: ST37 uses minimal alloying to preserve formability and weldability, while ST52 uses slightly higher alloying and/or microalloying plus controlled processing to raise yield strength.

Table: Typical compositional ranges (indicative; consult the applicable standard or mill certificate for exact numbers)

Element	ST37 (typical, DIN 17100 / S235-like)	ST52 (typical, DIN 17100 high-strength)
C (wt%)	up to ~0.17–0.22 (low)	up to ~0.20–0.25 (low to moderate)
Mn (wt%)	~0.50–1.40	~0.60–1.60
Si (wt%)	up to ~0.30–0.40	up to ~0.30–0.50
P (wt%)	≤ ~0.035–0.045	≤ ~0.035–0.045
S (wt%)	≤ ~0.035–0.045	≤ ~0.035–0.045
Cr (wt%)	usually trace	usually trace (not a primary hardener)
Ni (wt%)	usually trace	usually trace
Mo (wt%)	usually trace	possible trace if required
V (wt%)	typically absent	possible microalloying (~0.01–0.10)
Nb (wt%)	typically absent	possible microalloying (~0.01–0.06)
Ti (wt%)	typically absent	possible microalloying (~0.01–0.05)
B (wt%)	not typical	rarely used
N (wt%)	trace	trace

Notes: - Values above are indicative ranges; the DIN 17100 series allowed a number of subgrades and production routes. For procurement, always specify the exact standard revision, subgrade, and require mill test certificates. - ST52 strength is typically achieved by a combination of slightly higher carbon and manganese and by microalloying (Nb, V, Ti) and/or thermomechanical rolling rather than heavy alloy additions like Cr/Mo/Ni. - Alloying effects: C and Mn increase strength and hardenability but reduce weldability and ductility if high. Microalloying (Nb, V, Ti) promotes precipitation strengthening and finer ferrite-pearlite/grain size, enabling higher yield at lower carbon levels.

3. Microstructure and Heat Treatment Response

• ST37 microstructure: As-rolled ST37 typically exhibits a ferrite–pearlite microstructure with coarse ferrite grains relative to microalloyed steels. This microstructure gives good ductility and energy absorption.
• ST52 microstructure: ST52, depending on production, will commonly show a finer-grained ferrite–pearlite matrix with possible microalloy precipitates (NbC, VC, TiN) that restrain grain growth and provide precipitation strengthening. Thermomechanical controlled processing (TMCP) is often used to refine grain structure and enhance yield.
• Response to heat treatment:
• Normalizing: Both grades respond to normalizing with refined grain size and modest strength increase; ST52 benefits more from TMCP than simple normalizing.
• Quenching & tempering: Not typical for these steels in standard structural applications; quench & temper cycles produce much higher strengths but are used only when specified and with chemistry suitable for hardenability.
• Thermomechanical processing: ST52 is commonly produced with TMCP to achieve high yield with acceptable toughness and weldability; ST37 typically does not require TMCP.
• Practical implication: ST52 can achieve higher strength while maintaining acceptable toughness due to microalloying and processing, whereas ST37 obtains toughness and ductility from low-carbon chemistry and coarser microstructure.

4. Mechanical Properties

Table: Typical mechanical property ranges (as-rolled / supplier data; consult mill certificates)

Property	ST37 (typical)	ST52 (typical)
Yield strength (Rp0.2, MPa)	≈ 235 (common minimum for S235-like steels)	higher class — commonly in the 355–520 MPa region depending on subgrade and processing
Tensile strength (MPa)	≈ 360–510	≈ 500–700 (varies with subgrade and processing)
Elongation (%)	≈ 20–30 (good ductility)	≈ 10–20 (reduced ductility vs ST37)
Impact toughness	Often ≥ 27 J at room temp (JR) or specified temperature	Can be engineered to meet impact requirements; lower ductility can reduce toughness unless TMCP/microalloying is used
Hardness (HB)	Low-moderate (easy to machine/form)	Higher (harder to machine/form)

Explanation: - ST52 is the stronger grade; it achieves higher yield and tensile strength. That higher strength commonly comes at the expense of ductility (lower elongation) unless careful processing and microalloying are used to retain toughness. - Impact toughness depends heavily on specified subgrade and delivery condition (temperature, presence of J or JR suffixes). Both grades can be produced to meet specific impact energy requirements, but higher-strength variants often require attention to notch toughness.

5. Weldability

Weldability is influenced primarily by carbon content, carbon equivalent, and presence of microalloying elements which affect hardenability and susceptibility to hydrogen cracking.

Common weldability indices: - The IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The Pcm formula (for assessing cold-cracking susceptibility): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation (no numeric inputs here): - ST37: Low carbon and simplified chemistry produce low carbon-equivalent values, giving generally excellent weldability with minimal preheat and simple filler-metal choices. Hydrogen-assisted cracking risk is low for typical thicknesses. - ST52: Higher strength and the potential presence of microalloying elements increase the effective hardenability and raise carbon-equivalent-like indices. As a result, ST52 may require controlled preheat, interpass temperatures, and appropriate filler metal selection to avoid hard, brittle heat-affected zones and cold-cracking on thicker sections. - Practical guidance: For ST52, follow supplier recommendations for preheat, use matching or slightly overmatching fillers where specified, control hydrogen sources, and apply post-weld heat treatment (PWHT) when needed for critical applications.

6. Corrosion and Surface Protection

• Both ST37 and ST52 are carbon/low-alloy steels; neither is stainless. They rely on surface protection for corrosion resistance.
• Typical protection strategies: hot-dip galvanizing, zinc metallization, epoxy/polyurethane coatings, sacrificial coatings, or corrosion allowance in design.
• PREN (pitting resistance equivalent number) is relevant only for stainless and duplex stainless steels. For ST37 and ST52, PREN is not applicable: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Corrosion performance is largely independent of the slight compositional differences between ST37 and ST52; both require coatings in exposed environments. Surface preparation, coating system, and environment determine service life more than the small alloying differences.

7. Fabrication, Machinability, and Formability

• Cutting and machining:
• ST37: Easier to machine and cut due to lower hardness; tooling life and cutting forces are lower.
• ST52: Higher cutting forces and tool wear due to higher strength and hardness; machining parameters must be adjusted, and tooling may need upgrades.
• Bending and forming:
• ST37: Better formability and larger bend radii permitted for a given thickness; suitable for cold forming and rolling operations.
• ST52: Reduced ductility means tighter forming requires more care; springback is higher and minimum bend radii are larger unless material is specially processed.
• Surface finishing:
• Both accept standard finishing practices (grinding, shot-blasting, painting). ST52’s harder surfaces may require more robust finishing methods.

8. Typical Applications

Table: Typical uses

ST37 (typical uses)	ST52 (typical uses)
General structural steelwork (beams, columns) where high ductility and weldability are priorities	Structural members where higher strength-to-weight ratio is needed (reduced section thickness)
Fabricated frames, brackets, light to medium-load structures	Heavy machinery frames, cranes, load-bearing members with high stress
Pipe and tube for non-pressure applications, welded sections	High-strength pipes, pressure components when specified for strength (with welding controls)
General plates and sections for civil construction	Applications where weight reduction and higher yield are required (vehicle frames, offshore structural components when treated appropriately)

Selection rationale: - Choose ST37 when cost, easy forming and welding, and high ductility/toughness are primary drivers. - Choose ST52 when design calls for higher yield strength to reduce section thickness or to withstand higher static loads, provided fabrication controls for welding and forming are put in place.

9. Cost and Availability

• Cost: ST37 is generally less costly per tonne than ST52 because it contains fewer microalloying additions and requires less processing. ST52 often commands a premium due to microalloying and TMCP.
• Availability: Both grades are widely available in standard product forms (plate, sheet, hot-rolled coil, structural sections). ST37-like materials (S235) are perhaps more ubiquitous for simple construction steel, while ST52 equivalents may be specified and stocked for applications that require higher strength; lead times for certified, high-strength subgrades may be marginally longer.

10. Summary and Recommendation

Table: Quick comparison

Criterion	ST37 (lower-strength)	ST52 (higher-strength)
Weldability	Excellent	Good to moderate (requires controls)
Strength–Toughness balance	Lower strength, higher ductility/toughness	Higher strength, potentially lower ductility unless TMCP/microalloyed
Cost	Lower	Higher

Conclusion and selection guidance: - Choose ST37 if: - The application prioritizes weldability, easy forming, and high ductility (e.g., general structural steelwork, simple welded frames). - Cost sensitivity and abundant supply are important. - Impact toughness at ambient temperatures and ease of fabrication are required without advanced processing.

• Choose ST52 if:
• The design requires higher yield and tensile strength to reduce section sizes or support higher static loads.
• Weight reduction or higher load capacity is a design objective and the project can accept modestly higher material cost and fabrication controls.
• The purchaser specifies TMCP or microalloyed subgrades and appropriate welding/fabrication procedures are implemented.

Final note: ST37 and ST52 cover a spectrum of chemistry and processing states. Always specify the exact standard revision, delivery condition (e.g., normalized, TMCP), and required mechanical and impact properties in procurement documents. Require mill test certificates and, where applicable, weld procedure qualifications and preheat/PWHT instructions to ensure in-service performance matches design intent.