AZ100 vs AZ150 – Composition, Heat Treatment, Properties, and Applications

Introduction

AZ100 and AZ150 are designations commonly used in the metal-coating and sheet-steel supply chain to distinguish between two aluminium–zinc alloy coated steel options. Engineers, procurement managers, and manufacturing planners frequently face the choice between thinner and thicker Al–Zn alloy coatings: the trade-offs usually center on corrosion resistance versus cost, and formability versus coating durability. In practice, the principal technical distinction between AZ100 and AZ150 lies in the coating specification—principally coating mass (thickness) and the alloy balance of aluminium and zinc—rather than a fundamentally different base-steel metallurgy. That difference drives durability in corrosive environments, sacrificial protection performance, and some fabrication responses, so these variants are compared when projects require optimized life-cycle cost, surface performance, and formability.

1. Standards and Designations

Several international and national standards govern aluminium–zinc coated steels and the substrate steels they are applied to. Typical standards and documents to consult include: - ASTM/ASME: ASTM A792/A792M — specification for steel sheet, 55% Al–Zn alloy-coated by the hot-dip process (and related ASTM documents for substrate cold-rolled or hot-rolled steels). - EN: EN 10346 — continuously hot-dip coated steel flat products (specifies product families and coating types). - JIS: JIS standards addressing metallic-coated steel for building and industrial use (refer to the appropriate JIS for Al–Zn alloys and substrate steels). - GB: Chinese GB/T standards covering metallic-coated steel products and hot-dip coating mass/characteristics.

Classification note: AZ100 and AZ150 are coating/type designators (Al–Zn coated). The underlying steel is most often a carbon or low-alloy structural/forming steel (cold-rolled or hot-rolled). These coated products are not tool steels or stainless steels; they are typically carbon/low-alloy substrate steels with an aluminium–zinc protective coating applied.

2. Chemical Composition and Alloying Strategy

AZ coatings are aluminium–zinc alloys applied by continuous hot-dip processes. The protective performance results from a combination of barrier protection (aluminium-rich surface layer) and galvanic action (zinc contribution). The base steel chemistry is chosen to meet mechanical and forming requirements and is distinct from the coating chemistry.

Table: Typical compositional descriptors for substrate and coating (qualitative/representative)

Element	Substrate (typical carbon/low-alloy substrate)	AZ100 coating (representative)	AZ150 coating (representative)
C	Low carbon for formability; often ≤ ~0.12–0.20% for forming grades	—	—
Mn	Controlled for strength and hardenability; often 0.3–1.5% depending on grade	—	—
Si	Small amounts to aid deoxidation; substrate dependent	Minor addition to coating bath may be used to control wetting	Minor addition to coating bath; similar to AZ100
P, S	Kept low for ductility and surface quality	Trace impurities controlled	Trace impurities controlled
Cr, Ni, Mo, V, Nb, Ti, B	Present in microalloyed substrates as needed for strength or grain control	Typically not primary constituents of the Al–Zn coating	Same as AZ100; primary difference is coating mass and possible slight alloy balance adjustments
Al (coating)	N/A	Predominant alloying element in the coating — provides barrier property	Predominant; coating mass higher versus AZ100
Zn (coating)	N/A	Provides galvanic protection; relative balance with Al may vary slightly	Higher total zinc content per unit area due to increased coating mass

How alloying affects properties: - Substrate alloying (C, Mn, microalloying elements) controls intrinsic mechanical properties, hardenability, and grain size stability. - The Al–Zn coating composition and thickness control corrosion protection: aluminium contributes to barrier and adhesion, while zinc contributes sacrificial (galvanic) protection. A heavier coating mass increases lifetime in corrosive atmospheres; modest adjustments in the Al:Zn balance can tune barrier versus cathodic protection behavior.

3. Microstructure and Heat Treatment Response

Because AZ100 and AZ150 primarily designate coating differences, the microstructure of the steel substrate is the determining factor for heat-treatment response. Typical observations:

• Substrate microstructure: depending on alloy and processing, typical base microstructures are ferrite–pearlite for common cold-rolled/formable grades, and bainitic or tempered martensitic structures for higher-strength quenched-and-tempered or thermomechanically processed steels.
• Coating microstructure: the hot-dip Al–Zn coating solidifies to produce an aluminium-rich outer layer and an Al–Zn intermetallic region at the coating–substrate interface; intermetallic morphology and thickness are influenced by cooling rate and coating mass.

Effects of thermal processing: - Normalizing or annealing of the substrate prior to coating influences coating adherence and intermetallic development; coating is usually applied after any final annealing (or in-line processing). - Quenching & tempering or thermomechanical treatments applied to achieve high strength are conducted on the substrate prior to coating in most commercial routes; post-coating heat treatments that significantly raise temperatures can alter the coating microstructure and reduce performance. - Thicker coatings (AZ150) produce a larger intermetallic zone and a thicker outer alloy layer; excessive heat exposure post-coating can promote interdiffusion, potentially affecting adhesion and ductility at extreme conditions.

4. Mechanical Properties

Mechanically, AZ100 and AZ150 are similar because the coating contributes little to bulk tensile strength. However, coating mass can influence localized surface behavior, fatigue initiation, bending performance, and forming limits.

Table: Comparative mechanical property descriptors

Property	AZ100 (typical behavior)	AZ150 (typical behavior)
Tensile Strength	Determined by substrate; coating has negligible effect	Same as AZ100 (substrate-controlled)
Yield Strength	Substrate-controlled	Substrate-controlled
Elongation	Substrate-controlled; slightly better formability in very tight bends because of thinner coating	Slight reduction in ductility at extreme local strains due to thicker, less ductile coating
Impact Toughness	Substrate-controlled; coating thickness has minimal effect on bulk toughness	Same for bulk; surface-edge impact resistance may be marginally improved because of thicker sacrificial layer
Hardness (surface)	Slightly lower surface hardness than AZ150 due to lower coating mass	Slightly higher surface hardness and abrasion resistance owing to thicker metal layer

Interpretation: Choose substrate steel to meet strength and toughness requirements; select AZ100 when higher ease of forming and lower cost are priorities; select AZ150 when extended corrosion protection and surface durability are priorities. Differences in mechanical properties are mostly secondary effects of coating mass rather than intrinsic metallurgical differences.

5. Weldability

Weldability depends primarily on substrate chemistry (carbon, alloying, and microalloying) and on the coating’s effect on weld behavior (spatter, porosity, coating removal necessity).

Important carbon-equivalent and weldability indices to consider: - The IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The international Pcm parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - The coating itself can generate zinc vapor and porosity if welded without preparation; heavier coatings (AZ150) tend to generate more coating vapor and require more rigorous coating removal or special welding parameters in the weld zone than AZ100. - Substrates with low carbon and low CE values are more weldable; microalloying (Nb, Ti, V) requires attention for preheat/postheat depending on CE. - For resistance welding or spot welding, thicker coatings can affect electrode life and weld nugget formation; process parameters may need adjustment when moving from AZ100 to AZ150.

Practical guidance: For critical welded structures, specify weld preparation (stripping or using compatible filler metals), control CE of the substrate, and test-weld procedures for the specific coating mass.

6. Corrosion and Surface Protection

AZ coatings provide mixed protection modes: an aluminium-rich barrier and cathodic protection from zinc. The useful life in a given environment scales with coating mass and the operating conditions.

• For non-stainless substrates: corrosion mitigation is achieved by the alloyed Al–Zn coating, and additional protection can be provided by painting, conversion coatings, or sealants. AZ150, with greater coating mass, typically offers substantially longer service life in corrosive atmospheres than AZ100 because sacrificial zinc capacity and barrier thickness are higher.
• For stainless-like indices: PREN is not applicable to Al–Zn coatings or carbon substrates. PREN is relevant only for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Clarification: Do not use PREN for coated carbon steels; instead, use environmental exposure testing, salt-spray, cyclic corrosion tests, and field data to compare AZ100 vs AZ150 performance.

7. Fabrication, Machinability, and Formability

• Forming: Thinner coatings (AZ100) generally yield better performance in tight bending or deep drawing due to reduced risk of coating cracking, flaking, or powdering. AZ150 may require larger bend radii or process optimization (lower lubrication, appropriate bending speeds).
• Cutting and shearing: Both coatings are compatible with typical blanking, shearing, and laser processes, but thicker coatings can increase burring and require parameter tuning. Laser cutting can cause edge oxidation of the coating if not optimized.
• Machinability: The coating has limited effect on bulk machinability (substrate dominated). Surface finishing after forming (deburring, painting) should account for coating residue; AZ150 can require more aggressive surface prep for painting adhesion.
• Joining and fastening: Self-piercing rivets and mechanical fasteners perform similarly, but corrosion protection at fastener interfaces benefits from heavier coating mass.

8. Typical Applications

AZ100 — Typical uses	AZ150 — Typical uses
Building interior panels, non-critical roof underlayers, ductwork, trim where moderate corrosion protection and high formability are required	Exposed roofing and cladding in moderate to aggressive environments, industrial enclosures, agricultural equipment where extended corrosion life is required
Automotive inner panels and components where subsequent painting is applied and high formability is required	Coastal or chemical plant enclosures, long-life cladding, gutters and downpipes in aggressive atmospheres
Light structural applications where cost and bending performance are prioritized	Applications where sacrificial protection lifetime and abrasion resistance matter more than tight-radius forming

Selection rationale: choose the coating mass that matches environmental exposure and life-cycle cost. If subsequent coatings or paints will be applied, AZ100 may suffice; for exposed, unpainted surfaces, AZ150 often yields superior life expectancy.

9. Cost and Availability

• Cost: AZ150 typically carries a higher material cost than AZ100 due to increased metal usage in the coating bath and added processing. The cost difference should be evaluated on a life-cycle basis: higher initial cost often yields longer maintenance intervals.
• Availability: Both coating classes are commonly stocked in the market for common substrate steels. Availability by product form (coil, sheet, prepainted) varies by region and supplier; longer lead times may occur for specialty substrate grades coupled with specific coating masses.

10. Summary and Recommendation

Table summarizing key trade-offs

Metric	AZ100	AZ150
Weldability	Better in terms of lower zinc vapor for welding; still requires weld prep	Requires more attention to weld prep and parameters due to greater coating mass
Strength–Toughness (substrate)	Substrate-controlled; generally equivalent to AZ150	Equivalent; differences are surface-related rather than bulk
Cost	Lower initial material cost	Higher initial cost, longer service life in many conditions

Conclusions: - Choose AZ100 if you need superior formability, lower initial material cost, and the coated steel will be used in less aggressive environments or will be painted/overcoated after fabrication. - Choose AZ150 if the application demands extended corrosion protection for exposed surfaces, improved abrasion or sacrificial protection, and you accept a somewhat higher material cost and possible additional fabrication controls (welding, tight bending).

Final practical note: specify coating mass, substrate grade, and required exposure class explicitly in procurement documents; request manufacturer data sheets and independent corrosion-life or accelerated-test results for the precise AZ100 and AZ150 product you intend to use, because vendor alloy balance and process controls influence real-world performance as much as the nominal AZ designation.