Z25 vs Z35 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners often face the choice between two close product grades when specifying steel for structural, pressure-retaining, or heavy-fabrication work. The decision between Z25 and Z35 typically balances required strength and through-thickness performance versus cost, weldability, and availability. Common decision contexts include welded vessel shells, heavy plate for bridges, and fabricated structures where lamellar tearing (delamination) risk or directional toughness is a concern.

At a high level, Z35 is positioned as the higher-performance grade compared with Z25: it generally offers greater through-thickness resistance to layered tearing and higher strength, often achieved by controlled chemistry and thermo-mechanical processing. Z25 is selected where adequate strength and toughness are required at lower cost and with easier fabrication and welding. These grades are compared because they target similar application domains but differ in alloying strategy and processing to optimize the trade-offs above.

1. Standards and Designations

• Major standards where Z-prefixed or similarly numbered product grades may appear: national and proprietary designations (EN, ASTM/ASME, JIS, GB) or mill standards. Note: Z25 and Z35 are product-grade labels used by some mills and specifiers; they are not universal ASTM names like S275 or S355.
• Classification by family:
• Z25: typically a low-alloy or microalloyed structural steel (low- to mid-strength range).
• Z35: typically a higher-strength microalloyed or low-alloy steel optimized for improved through-thickness behavior and higher yield/tensile strength.
• Users must map Z25/Z35 to the specific standard specification or supplier mill certificate for procurement and design verification.

2. Chemical Composition and Alloying Strategy

The exact chemistry of Z25 and Z35 is supplier-specific. The table below gives typical alloying elements of steels in this performance bracket and qualitative or indicative ranges common in mill product data. Always verify with the certificate of analysis.

Table: Typical composition ranges (indicative; consult mill certificate) | Element | Z25 (typical strategy) | Z35 (typical strategy) | |---|---:|---:| | C (carbon) | Low to medium; optimized for weldability and ductility (indicative: ~0.08–0.20%) | Low to medium; controlled to balance strength and weldability (indicative: ~0.08–0.22%) | | Mn (manganese) | Moderate to aid strength and hardenability (indicative: ~0.3–1.2%) | Moderate to slightly higher to increase strength (indicative: ~0.4–1.4%) | | Si (silicon) | Small amounts for deoxidation (≈0.1–0.4%) | Similar, controlled for toughness (≈0.1–0.4%) | | P (phosphorus) | Kept low for toughness (<0.03%) | Kept low for toughness (<0.03%) | | S (sulfur) | Kept low for ductility (<0.02%) | Kept low for ductility (<0.02%) | | Cr, Ni, Mo (alloying) | Usually minimal or absent in basic grades; may contain small additions in alloyed variants | May contain small controlled additions to improve hardenability and toughness in variant grades | | V, Nb, Ti (microalloying) | May include trace microalloying (ppm levels) to refine grain and raise strength | More likely engineered microalloy content and processing to improve through-thickness toughness | | B (boron) | Typically absent or at very low levels | May be used in minute additions by some mills to enhance hardenability (ppm) | | N (nitrogen) | Controlled to manage inclusion stability and strength | Controlled; low N often aids toughness |

How alloying affects properties: - Carbon and manganese raise strength and hardenability but can reduce weldability and toughness if excessive. - Microalloying elements (Nb, Ti, V) allow strength increase via grain refinement and precipitation hardening without large increases in carbon. - Controlled low interstitials (P, S, N) and clean steel practice improve ductility and through-thickness performance.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses:

• Z25:
• Process routes: conventional hot rolling with optional normalizing or light tempering.
• Microstructure: predominantly ferrite–pearlite or fine-grained ferrite with controlled pearlite; microalloyed variants show fine precipitates that raise yield strength.

• Heat treatment response: normalizing refines grain and improves toughness; heavy quench & temper treatments are uncommon for this class.

• Z35:

• Process routes: may use controlled rolling (thermo-mechanical controlled processing, TMCP) and accelerated cooling to refine microstructure and promote fine bainitic/ferritic constituents.
• Microstructure: finer-grained ferrite with dispersed bainite or tempered martensite/bainite pockets in some low-alloy variants; engineered inclusion control to reduce lamellar tearing risk.
• Heat treatment response: TMCP and controlled cooling increase strength and improve through-thickness toughness more effectively than simple normalizing; quench & temper is possible if specified but changes classification.

Effect of common routes: - Normalizing: refines grain size and homogenizes microstructure; improves toughness for both grades. - Quenching & tempering: raises strength substantially but requires more stringent weld and pre-heat/post-heat controls. - TMCP: allows higher strength and toughness with good weldability by refining microstructure without heavy alloying.

4. Mechanical Properties

Exact mechanical values depend on mill certification, product form (plate, coil, forged part), and heat treatment. The following table gives indicative comparative ranges and qualitative descriptors; verify specific values in procurement documents.

Table: Indicative mechanical properties (typical ranges; consult mill data) | Property | Z25 (indicative) | Z35 (indicative) | |---|---:|---:| | Tensile strength | Moderate — typically in the low-to-mid range for structural steels | Higher — typically above Z25, reflecting higher proof strength | | Yield strength | Lower/mid range (suitable for general structural) | Higher; designed for higher static loads | | Elongation (%) | Good ductility; adequate for forming and fabrication | Slightly lower or comparable; depends on processing | | Impact toughness (Charpy, - or specified temp) | Moderate; depends on cleanliness and processing | Higher through-thickness and directional toughness; engineered to resist lamellar tearing | | Hardness (HB or HRC) | Moderate | Higher, but still within weldable bands for many grades |

Which is stronger, tougher, or more ductile, and why: - Strength: Z35 is engineered to provide higher yield and tensile strength than Z25 through a combination of slightly higher alloy content and process control. - Toughness: Z35 tends to deliver improved through-thickness and delamination resistance because of cleaner steel practices, inclusion shape control, and thermo-mechanical processing. This is crucial where layered tearing risk exists. - Ductility: Z25 can show marginally higher uniform elongation in some conditions due to lower strength; however, carefully processed Z35 can retain good ductility while increasing strength.

5. Weldability

Weldability is guided primarily by carbon equivalent and hardenability. For qualitative assessment you can use the IIW carbon equivalent and the Pcm index:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Z25: With relatively low carbon and simpler chemistry, tends to have a lower $CE_{IIW}$ and $P_{cm}$—resulting in easier welding, less preheat/postheat requirement, and lower susceptibility to cold cracking. - Z35: Higher strength and added microalloying or slightly higher Mn can increase $CE_{IIW}$ and $P_{cm}$ marginally. This necessitates more careful welding procedure specifications (PQR/WPS), possible preheating, and attention to hydrogen control. - In both grades: thickness, joint configuration, and fabrication practices (cleanliness, hydrogen bake-out, electrode selection) significantly influence weld performance. Verify with weld procedure qualification using the target plate chemistry.

6. Corrosion and Surface Protection

• Non-stainless grades: Z25 and Z35 are typically non-stainless low-alloy steels. Corrosion protection is provided by coatings and design:
• Hot-dip galvanizing, zinc-rich primers, epoxy coatings, or metallizing are common protective strategies.
• Select coating system based on environment (C3–C5 corrosivity categories or marine vs. rural/industrial).
• Stainless considerations: PREN is not applicable unless the grade is a stainless alloy. For reference, stainless alloys use: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ —used to estimate pitting resistance of stainless steels.
• When to consider stainless: If long-term corrosion resistance without external coatings is needed, move to stainless family rather than relying on Z25/Z35.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades can be flame-cut, plasma-cut, or laser-cut; Z35’s higher strength can require adjusted cutting parameters and produce harder heat-affected zones.
• Machinability: Z25 often machines slightly easier due to lower strength and less precipitation hardening. Z35, with microalloying and higher strength, can be tougher on tool wear; choose tooling and speeds accordingly.
• Formability and bending: Z25 generally offers better bendability at given thickness due to lower yield. Z35 can be formed if designed appropriately, but minimum bend radii may be larger and springback greater.
• Heat input and forming: For heavy forming or post-weld forming, consider the grade’s specified heat treatment and ensure processes do not induce unwanted strength loss.

8. Typical Applications

Table: Typical uses for each grade | Z25 (typical uses) | Z35 (typical uses) | |---|---| | General structural plate and beams where standard strength and good weldability are priority | Heavy fabricated components and plates where higher strength and improved through-thickness performance are required | | Fabricated frames, conveyors, and general steelwork | Pressure vessel skirts, heavy flanges, and welded structures with risk of lamellar tearing | | Medium-duty welded tanks and bins with coatings for corrosion protection | Bridges, offshore or nearshore structural elements where through-thickness toughness is critical | | Applications prioritizing cost, ease of procurement, and straightforward welding | Applications prioritizing higher load capacity, fatigue resistance, and reduced delamination risk |

Selection rationale: - Choose Z25 when cost, straightforward fabrication, and good overall ductility/weldability are primary drivers. - Choose Z35 when stronger, tougher plate with engineered through-thickness performance is required (e.g., thick welded joints, heavy load-bearing welded assemblies).

9. Cost and Availability

• Cost: Z35 typically carries a premium over Z25 because of tighter chemistry control, microalloying, TMCP processing, or additional qualification steps. The premium varies by region and producer.
• Availability: Z25 is often more widely available in multiple mill sizes, thicknesses, and product forms. Z35 may be available as standard plate coils and plates but can be limited in specialty thicknesses or in small-batch orders; lead times can be longer for certified Z35 material.

10. Summary and Recommendation

Table summarizing key trade-offs | Characteristic | Z25 | Z35 | |---|---:|---:| | Weldability | High; easier PQR/WPS | Good but more attention required to preheat/hydrogen control | | Strength–Toughness balance | Moderate strength with good ductility | Higher strength with improved through-thickness toughness | | Cost | Lower | Higher |

Recommendations: - Choose Z25 if: - Your project prioritizes cost-effectiveness, straightforward welding, and general structural performance. - Plate thickness, joint configuration, and service conditions do not present elevated lamellar tearing or through-thickness stress risks. - You require broad availability and short lead times.

• Choose Z35 if:
• You need higher yield/tensile strength and enhanced resistance to layered or through-thickness tearing.
• The design includes heavy welded connections, thick plates, or conditions where directional toughness is important (fatigue-prone or cyclic loading).
• You accept a higher material cost and possibly stricter fabrication controls for a more durable structural outcome.

Final note: Z25 and Z35 are shorthand product-grade labels whose exact chemistry and mechanical guarantees must be obtained from the mill certificate and the applicable specification. For safety-critical or welded pressure-containing applications always specify required mechanical properties (yield, tensile, impact at temperature), maximum permitted chemistry values, and required weld procedures in procurement and drawings.