StW22 vs StW24 – Composition, Heat Treatment, Properties, and Applications

Introduction

StW22 and StW24 are closely related structural carbon steels commonly considered for cold-formed and welded components in mechanical and structural applications. Engineers, procurement managers, and manufacturing planners frequently weigh trade-offs among cost, strength, formability, and weldability when selecting between them. Typical decision contexts include choosing a grade for deep-drawn parts, welded assemblies, or heavier-load structural members where forming limits and post-processing costs matter.

The principal differentiator between StW22 and StW24 lies in their mechanical balance: StW24 is formulated and processed to achieve slightly higher strength and hardenability, while StW22 is tailored to provide greater ductility and stretch-forming capacity. Because both grades occupy adjacent positions in the same family, selection often depends on whether the project prioritizes higher elongation/formability or modest increases in strength with similar manufacturability.

1. Standards and Designations

• Typical standards where “StW” style designations appear are national and regional standards derived from older German/DIN naming conventions and equivalents in EN/ISO and other national systems. Modern specifications may map these legacy names to specific EN or ISO designations; always verify the exact standard edition when procuring material.
• Classification:
• StW22 — unalloyed low-carbon structural steel for cold forming (carbon steel).
• StW24 — low-alloy or microalloyed structural steel with slightly higher strength/hardenability than StW22 (generally still treated as carbon/low-alloy steel).
• Note: For procurement, confirm the applicable standard (e.g., supplier’s datasheet, EN standard number) rather than relying solely on legacy grade names. Some national standards or mills may use similar labels with small compositional or property differences.

2. Chemical Composition and Alloying Strategy

The following table summarizes the usual alloying strategy for the two grades in qualitative terms (presence and role of elements rather than exact values). For project-critical selection, request the mill certificate for precise wt% values.

Explanation: - Both grades are low-carbon steels; their mechanical behavior is primarily tuned by controlling carbon and manganese levels and by small amounts of microalloying elements in StW24 to boost strength without excessive loss of ductility. - Microalloying (V, Nb, Ti) is a common strategy to increase yield strength through precipitation strengthening and grain refinement rather than by raising carbon, which would harm weldability and formability.

3. Microstructure and Heat Treatment Response

Typical microstructures depend on processing route (hot-rolled, cold-rolled, normalized, or thermomechanically rolled):

• StW22:
• As-rolled or annealed condition: primarily ferritic with pearlitic islands if carbon is present; fine ferrite dominates due to low carbon.
• Response to normalizing: more uniform ferrite-pearlite mix with slightly refined grains; retains high ductility.
• Quenching & tempering: not commonly applied because the grade is optimized for formability rather than high hardened strength.

• Thermo-mechanical controlled processing (TMCP): refines grain size, improving toughness without losing ductility.

• StW24:

• As-rolled: similar ferrite-pearlite but with slightly higher pearlite or bainitic constituents if microalloyed or if cooling rates are higher.
• TMCP or microalloying leads to finer-grained ferritic matrix with dispersed carbides/nitrides, increasing yield strength and hardenability.
• Quench & tempering can be used on select variants to reach higher strength classes, but basic commercial StW24 is often supplied in hot-rolled or cold-rolled delivered conditions.

Practical implication: StW24’s composition and processing allow modest increases in strength and hardenability at similar heat-treatment complexity, while StW22 tends to preserve elongation and forming capability.

4. Mechanical Properties

Because actual property numbers depend on exact chemistry and processing, the following table presents typical comparative behavior (qualitative).

Explanation: - StW24 is engineered to give a modest increase in tensile and yield strength, often at the cost of ultimate elongation. The net change is intentional to allow lighter sections or smaller components while maintaining manufacturability. - Impact toughness at room temperature is generally acceptable for both when produced and processed properly; low-temperature toughness must be confirmed by supplier test reports.

5. Weldability

Weldability is influenced by carbon content, hardenability-inducing elements, and residuals. Two common empirical indicators are the IIW carbon equivalent and the Pcm formula:

• Use the IIW carbon equivalent for a quick assessment: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$

• For thicker sections and structural weldability assessment: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn + Cu}{20} + \frac{Cr + Mo + V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Both grades typically have low carbon and controlled alloying, giving favorable weldability for common fusion and resistance welding procedures. - StW22, with slightly lower hardenability-inducing elements, generally offers easier welding with minimal preheat requirements and lower risk of cold cracking. - StW24, due to slightly higher Mn and possible microalloying, can have higher hardenability; for thick sections or high restraint joints, preheat or controlled heat input may be advisable. Use supplier-specified preheat and post-weld heat treatment guidance and check hydrogen control and consumable selection. - Practical recommendation: when in doubt, perform welding trials and consult the mill’s weldability data and the CE/Pcm calculations using actual composition data from the certificate.

6. Corrosion and Surface Protection

• These grades are not stainless steels; corrosion resistance is typical of plain carbon steels.
• Standard protection methods:
• Hot-dip galvanizing for outdoor structural exposure.
• Organic coatings (primers and topcoats) for aesthetic and moderate corrosion environments.
• Protective conversion coatings and cathodic protection where appropriate.
• PREN (pitting resistance equivalent number) is not applicable to non-stainless structural steels, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• This index applies to stainless families (austenitic/duplex), not to StW steels.
• For atmospheric or mildly corrosive environments, selection depends on exposure class and life-cycle cost: galvanizing or painting is typically sufficient; for marine or chemical exposure, a stainless grade or additional protection should be selected.

7. Fabrication, Machinability, and Formability

• Formability:
• StW22: superior stretch-forming and deep-drawing performance owing to higher elongation; preferred for complex shapes and severe forming operations.
• StW24: capable of forming but with more limited true-strain capacity; springback may be different due to higher yield.
• Machinability:
• Both are generally machinable in the annealed or normalized condition. Slightly higher strength in StW24 may increase tool wear and require higher cutting forces.
• Sulfurized free-cutting variants are a different product family; standard StW grades prioritize forming over machinability additives.
• Finishing:
• Surface quality, pickling/cleaning, and any coating adhesion need the same attention for both grades. Heat treatment or cold-work can affect hardness and thus finishing parameters.

8. Typical Applications

Selection rationale: - Choose StW22 when forming complexity and elongation are critical and load levels are moderate. - Choose StW24 when you need to reduce cross-section or mass for a given load, and when forming requirements are moderate-to-mild.

9. Cost and Availability

• Cost: StW24 is typically slightly more expensive than StW22 due to tighter chemistry control and possible microalloying or TMCP processing. The incremental cost varies by mill, order volume, and product form.
• Availability: Both grades are commonly offered as hot-rolled and cold-rolled coils/sheets and as structural plate from major suppliers. Availability in exact standardized forms depends on regional supply chains and whether modern EN/ISO equivalents are specified; check lead times for specific thicknesses and surface conditions.

10. Summary and Recommendation

Choose StW22 if: - Your design requires maximum formability, deep drawing, or complex bending and stretch forming. - Welded assemblies are simple and you prefer easier welding with minimal preheat. - Cost sensitivity and ease of fabrication are prioritized over a marginal strength increase.

Choose StW24 if: - You need a modest increase in tensile and yield strength to reduce section size or achieve weight savings. - Fabrication and forming requirements are moderate and can tolerate slightly reduced elongation. - You accept a slightly higher material cost in exchange for improved strength-to-weight potential.

Final note: Always verify the exact chemical and mechanical certification from the mill for the batch you intend to use. Small compositional differences and processing routes (annealed, normalized, TMCP) have material effects on forming, welding, and service performance; when in doubt, request sample coupons for forming, welding, and mechanical testing before committing to full production.