18-8 स्टेनलेस स्टील: संघटन, प्रदर्शन & उपयोग

1. कार्यकारी सारांश

“18-8 stainless steel” is the common name for a family of austenitic stainless steels characterized by roughly 18% क्रोमियम और 8% निकल (hence “18-8”).

The best-known member is प्रकार 304 (US S30400 / में 1.4301). 18-8 alloys are the workhorses of stainless technology because they combine broad corrosion resistance, उत्कृष्ट निर्माण, उच्च क्रूरता, and simple fabrication.

They are not, तथापि, the best choice for aggressive chloride environments or high-temperature creep applications — in those cases alloys with added molybdenum, stabilized or duplex microstructures, or nickel-base alloys are preferred.

2. What “18-8” means — definition and scope

“18-8” is an informal, historical descriptor that designates स्टेनलेस स्टील्स साथ लगभग 18 wt.% chromium और 8 wt.% nickel—the classic austenitic stainless composition introduced in the early 20th century.

It typically refers to the 300-series austenitic परिवार: principally प्रकार 304 and its variants (304एल, 304एच), plus related stabilized grades (उदा।, 321, 347) that share the 18–20% Cr / 8–10% Ni base but add titanium or niobium to control carbide precipitation.

प्रमुख बिंदु:

• “18-8” is a practical shorthand — specify exact grade (उदा।, 304, 304एल, 321) in procurement.
• The austenitic microstructure is stabilized by Ni; Cr provides passivity and oxidation resistance.

3. Typical grades and standards

Common commercially used 18-8 variants include:

• प्रकार 304 (US S30400 / में 1.4301) — standard 18-8 स्टेनलेस; सामान्य प्रयोजन.
• टाइप 304L (एस30403 / 1.4306) — low-carbon variant (≤0.03% सी) to reduce sensitization during welding.
• Type 304H (S30409 / 1.4307) — higher carbon (≈0.04–0.10%) for improved strength at elevated temperatures.
• प्रकार 321 (S32100 / 1.4541) — Ti-stabilized for better resistance to intergranular corrosion after exposure in 450–850 °C range.
• प्रकार 347 (एस34700 / 1.4550) — Nb-stabilized equivalent to 321.

Standards covering these grades include एएसटीएम ए240 / A240M (थाली, चादर), एएसटीएम ए276 (बार -बार), ASME/ASME II, and EN/ISO equivalents. Always reference the precise standard and UNS/EN number in specifications.

4. की रासायनिक संरचना 18-8 स्टेनलेस स्टील

5. Mechanical properties of 18-8 स्टेनलेस स्टील

The table below gives representative mechanical properties for typical 18-8 ऑस्टेनिटिक स्टेनलेस स्टील्स (उदा।, प्रकार 304 परिवार) in the solution-annealed / घिसी हुई अवस्था.

6. भौतिक & थर्मल विशेषताएं

• घनत्व: ≈ 7.9 जी·सीएम⁻³.
• Modulus of elasticity (ईटी): ≈ 193–200 GPa.
• ऊष्मीय चालकता: relatively low for a metal, ≈ 14–16 W·m⁻¹·K⁻¹ पर 100 ° C (falls with temperature).
• थर्मल विस्तार का गुणांक: ≈ 16–17×10⁻⁶ K⁻¹ (20-100 ° C) — higher than carbon steel, important for thermal joint design.
• पिघलने की सीमा: ठोस ~ 1375-1400 डिग्री सेल्सियस, तरल ~ 1400-1450 डिग्री सेल्सियस (composition dependent).
• Magnetic behavior: essentially गैर चुंबकीय in annealed condition; cold work or formation of martensite imparts mild ferromagnetism.

Temperature service limits: continuous use up to ~400–800 °C is possible depending on alloy and environment; beware of sensitization zone (~425–850 °C) and carburization/oxidation at high temperatures.

For sustained high-T strength consider 304H, 309, 310 or other high-temperature alloys.

7. Corrosion behaviour — strengths and limitations

ताकत

• अच्छा सामान्य संक्षारण प्रतिरोध in oxidizing atmospheres and many chemicals (acids/bases) at ambient temperatures.
  The passive Cr₂O₃ film grants broad utility in food, architectural and many process environments.
• Good hygiene and cleanability, which is why 18-8 is widely used in food, beverage and medical equipment.

सीमाएँ

• क्लोराइड में गड्ढे और दरारों का क्षरण: without Mo, 18-8 is susceptible to localized attack in chloride-bearing media (समुद्री जल, नमकीन) especially at elevated temperatures or in crevices.
  If chlorides are present, प्रकार 316 (with Mo) or duplex alloys are often chosen.
• तनाव संक्षारण दरार (एस सी सी): austenitic 18-8 steels are susceptible to chloride-induced SCC under tensile stress and elevated temperature; avoid combination of tensile stress + क्लोराइड + तापमान.
• अंतर कणीय क्षरण (संवेदीकरण): occurs after exposure to 425–850 °C unless low-C (304एल) या स्थिर ग्रेड (321/347) are used.
• गैल्वेनिक संक्षारण: when coupled to more noble alloys, 18-8 can act as an anode in certain electrolytes — design to avoid dissimilar metal contact or provide insulation.

Practical selection rule: For general service where chlorides or heavy reducing conditions occur, evaluate 316 (एमओ), super-austenitics, डुप्लेक्स या निकल मिश्र धातु.

8. छलरचना: गठन, मशीनिंग, welding and joining

बनाने

• उत्कृष्ट सूत्रीकरण in annealed condition due to high ductility. Use proper tooling to account for springback (higher than mild steel) and the strong work-hardening behavior.
• Deep drawing & कताई are common for cookware and thin-wall vessels.

मशीनिंग

• Notoriously “gummy” compared with carbon steel; austenitic stainless steels work-harden in the cut, which increases tool wear. Best practice:

• • Use rigid tooling, positive rake carbide tools.
  • Employ moderate cutting speeds, high feed for roughing, and abundant coolant to avoid built-up edge and heat.
  • Use sharp edges and chip breakers.

वेल्डिंग & में शामिल होने

• उत्कृष्ट वेल्डेबिलिटी by common methods (GTAW, GMAW, SMAW, एफ.सी.ए.डब्ल्यू). प्रमुख बिंदु:

• • Use low-carbon (304एल) for welded assemblies where post-weld sensitization is a concern.
  • Use appropriate filler metals (उदा।, 308L/308 stainless filler for 304 base metal) to match chemistry and avoid hot cracking.
  • ताप इनपुट को नियंत्रित करें & interpass temperature; excessive heat widens the sensitized zone.
  • Post-weld solution anneal (1050-1100 डिग्री सेल्सियस) followed by rapid quench can restore corrosion resistance where practical; often not feasible for assembled structures.
    वैकल्पिक, use low-C or stabilized grades to avoid the need for PWHT.
  • Beware of solidification cracking in some weld configurations — follow qualified WPS and prequalified procedures.

Other joining

• टांकना, टांकने की क्रिया, चिपकने वाला संबंध are used with appropriate fluxes and surface preps. Adhesive bonding frequently requires surface activation (ज्योति, प्लाज्मा, chemical etch).

9. उष्मा उपचार & thermal processing

• Not hardenable by quench & गुस्सा (austenitic 18-8 does not form martensite through heat treatment like carbon steels).
• समाधान की घोषणा: typical at 1010–1120 °C followed by rapid quench (पानी) to dissolve carbides and restore corrosion resistance and ductility. Used after welding/heavy cold work when feasible.
• Stress relief anneal: limited benefit; if performed, avoid temperatures in the sensitization range unless followed by solution anneal.
• बुढ़ापा: prolonged exposure to 475 ° C (475 °C embrittlement) in some iron-nickel-chromium alloys can embrittle the material — not typical for 304, but be cautious in long-time exposures.

10. सतह समापन, passivation and cleaning

• Mechanical finishes: 2बी, BA, No.1, No.4 (ब्रश) वगैरह. Select finish for application: polished for sanitary, matte for architectural.
• नमकीन बनाना & अदा करना: chemical pickling removes heat tint and embedded iron; अदा करना (nitric or citric acid treatments) restores and strengthens the passive film—critical after welding or fabrication.
  Citric acid passivation is increasingly preferred for safety and environmental reasons.
• Electropolishing: reduces surface roughness and improves corrosion resistance (useful in pharmaceutical/food industries).
• सफाई: avoid chlorinated cleaners; prefer mild alkaline cleaners or detergents followed by potable water rinse. For critical sanitary use, validate cleaning regimen.

11. के विशिष्ट अनुप्रयोग 18-8 स्टेनलेस स्टील

• Food service and processing equipment: डूब, कन्वेयर, tanks — hygienic, easily cleaned.
• Architectural surfaces and trim: टिकाऊ, corrosion-resistant finishes.
• Household goods: कटलरी, कुकवेयर, उपकरण पैनल.
• Chemical process equipment (mild services): पाइपलाइन, valves for non-chloride environments.
• फास्टनर, स्प्रिंग्स (when cold-worked), इंस्ट्रुमेंटेशन: using work-hardening for mechanical function.
• Medical devices and implants (select grades, controlled manufacturing): because of biocompatibility and sterilizability (but not all 18-8 variants are medical-grade).

12. Comparison to Related Alloys

13. निष्कर्ष

18-8 स्टेनलेस स्टील represents one of the most balanced and widely adopted material systems in modern engineering.

By combining approximately 18% क्रोमियम और 8% निकल, it achieves a stable austenitic microstructure that delivers an exceptional blend of corrosion resistance, यांत्रिक विश्वसनीयता, प्रपत्र, और वेल्डेबिलिटी.

These characteristics explain its long-standing dominance across food processing, रासायनिक उपकरण, architectural structures, दबाव वाहिकाओं, and general industrial applications.

 

पूछे जाने वाले प्रश्न

What does “18-8” mean in stainless steel?

“18-8” refers to the nominal chemical composition of approximately 18% क्रोमियम और 8% निकल.

This composition stabilizes an austenitic structure, providing corrosion resistance, लचीलापन, and non-magnetic behavior in the annealed condition.

है 18-8 stainless steel the same as Type 304?

प्रकार 304 is the most common standardized grade within the 18-8 परिवार.

While “18-8” is a general industry term, प्रकार 304 (and its variants such as 304L and 304H) represents a precisely defined specification under international standards.

है 18-8 stainless steel magnetic?

In the solution-annealed condition, 18-8 stainless steel is essentially non-magnetic. तथापि, cold working can induce partial martensitic transformation, resulting in slight magnetic response.

What are the main advantages of 18-8 stainless steel over duplex stainless steels?

18-8 stainless steel offers superior formability, easier welding, better low-temperature toughness, and lower material and fabrication costs.

Duplex stainless steels provide higher strength and improved chloride resistance but are more demanding to process.