High-performance super austenitic stainless steel: 904L (N08904)

Summary: 904L Stainless Steel

904L stainless steel, Unified Numbering System (UNS) designation N08904, is a super-austenitic stainless steel developed for resisting sulfuric acid corrosion, characterized by high molybdenum, high chromium, and high nickel content. Thanks to its exceptional corrosion resistance, excellent mechanical and physical properties, it holds an irreplaceable position in harsh industrial environments. This document will introduce its various properties, included standards, and application fields.

I. Material Introduction

904L (UNS N08904) is a low-carbon, high-alloy austenitic stainless steel. Its core design philosophy is to achieve superior corrosion resistance compared to standard stainless steels like 304 and 316 by increasing the alloy content. Its most notable features are low carbon content (≤0.020%) and high contents of molybdenum (4.0-5.0%), chromium (19.0-23.0%), nickel (23.0-28.0%), with the addition of copper (1.0-2.0%). This composition grants it outstanding resistance to sulfuric acid, phosphoric acid, and chloride stress corrosion cracking.

II. Chemical Composition

The chemical composition significantly affects the steel's metallographic structure, mechanical properties, physical properties, and corrosion resistance. (Per ASTM A240/A240M)

• C (Carbon): The low carbon content greatly reduces the risk of carbide precipitation at grain boundaries, avoiding intergranular corrosion.

• Cr (Chromium): Forms a passive film, providing basic resistance to corrosion by oxidizing media.

• Ni (Nickel): An austenite-forming element that stabilizes the austenitic phase, improving resistance to reducing media and stress corrosion cracking.

• Mo (Molybdenum): Significantly enhances resistance to pitting and crevice corrosion, especially in environments containing chloride ions.

• Cu (Copper): Improves corrosion resistance in reducing acids, such as in certain mixtures of sulfuric and phosphoric acids. It is a specialized alloying element added to high-performance austenitic stainless steels for these environments.

• Mn (Manganese): An austenite-forming element that improves hot workability.

• Si (Silicon): Improves oxidation resistance.

• N (Nitrogen): Can increase strength and pitting corrosion resistance.

III. Mechanical Properties

Compared to standard 300 series austenitic stainless steels like 304, the high-performance austenitic stainless steel 904L offers higher yield strength and tensile strength. This is largely due to the solid solution strengthening effect of C and N, and is directly related to the high degree of alloying with substitutional elements like Mo, Ni, and Cr. It is important to note that austenitic stainless steels cannot be hardened or have their strength increased by heat treatment. Their strength can be significantly increased through cold working processes. (Properties defined in standards like SA-240/SA-240M).

IV. Physical Properties

The physical properties of austenitic stainless steels are generally similar. Compared to carbon steel, they have a slightly lower Young's modulus of elasticity, a higher coefficient of thermal expansion, and lower thermal conductivity.

V. Corrosion Resistance

While the corrosion resistance of standard austenitic stainless steels meets the needs of many environments, high-performance austenitic stainless steels offer the strongest corrosion resistance among all stainless steels. They perform excellently in uniform corrosion, pitting and crevice corrosion, as well as stress corrosion cracking.

• Localized Corrosion: The high molybdenum and chromium content gives 904L a high Pitting Resistance Equivalent Number (PREN), calculated as PREN=%Cr + 3.3×%Mo + 16×%N. Its PREN value is 33-35, making it highly resistant to pitting and crevice corrosion caused by chlorides.

• Intergranular Corrosion: The extremely low carbon content enables effective resistance to intergranular corrosion caused by chromium carbide precipitation, even after short-term exposure to the sensitization temperature range (450~850°C) during welding.

• Stress Corrosion Cracking (SCC): The high nickel content provides much higher resistance to chloride stress corrosion cracking compared to standard austenitic stainless steels (e.g., 304L, 316L).

VI. Heat Treatment

904L must undergo solution heat treatment to achieve optimal corrosion resistance.

• Solution Treatment: Heat to 1100~1170°C, followed by rapid water quenching. The purpose is to fully dissolve all alloying elements into the austenitic matrix, obtain a uniform single-phase structure, and dissolve any carbides or intermetallic phases that may have formed during processing.

• Precautions: Prolonged停留 (dwelling) in the temperature range of 540~930°C should be avoided to prevent the precipitation of harmful phases (such as sigma phase), which can reduce toughness and corrosion resistance.

VII. Applications

904L is one of the most common alloys in the 4% molybdenum grade. It was developed as an upgrade for 316L and Alloy 20, while being less expensive than Alloy C276.

• Application Industries: Chemical; Oil and Gas; Pharmaceutical; Power Generation; Marine Engineering; Food Processing.

• Fasteners: Can be used to manufacture bolts, screws, nuts, studs, venting products, washers, pins and keys, pipe fittings.

VIII. Approximate Grade Cross-Reference and Near-Matching Grades

• Approximate Grade Cross-Reference: This refers to a table comparing different names and steel numbers for similar materials across various national or organizational standards. Typically, these grades can be substituted, but selection should be verified based on the actual application.

• Near-Matching Grades: This serves as a supplement to the cross-reference table, conditionally expanding the range of material alternatives. When using this data, it should be carefully evaluated based on the specific circumstances or in consultation with a materials engineer.

IX. Standards Including 904L

(This section would list the specific standards, e.g., ASTM A240, ASME SA-240, etc.)