Introduction to stainless steel types and surface treatment methods

Scope of Application Stainless steel is renowned for its unique strength, exceptional wear resistance, superior corrosion resistance, and minimal rusting properties. These characteristics make it a popular choice across various industries, including chemical engineering, food machinery, electrical and mechanical sectors, environmental protection, home appliances, interior decoration, and fine art industries. Its use imparts a sense of elegance and sophistication. The future potential for stainless steel applications appears limitless, yet the advancement of these applications heavily depends on the level of innovation in its surface treatment techniques. Proper surface treatments can significantly enhance the durability and aesthetic appeal of stainless steel, making it even more versatile. Approaches to Surface Treatment 1. Common Surface Treatment Techniques 1.1 Overview of Stainless Steel Varieties Stainless steel typically contains premium metallic elements like chromium (Cr), nickel (Ni), molybdenum (Mo), and titanium (Ti). The most common types include chrome stainless steel, which contains Cr ≥ 12%, and nickel-chrome stainless steel, which includes Cr ≥ 18% and Ni ≥ 12%. These materials can be classified based on their metallographic structures into austenitic stainless steel (e.g., 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5) and martensitic stainless steel (e.g., Cr17, Cr28). Austenitic types are usually non-magnetic, while martensitic ones are magnetic. 1.2 Standard Surface Treatment Methods Surface treatments commonly include natural color whitening, mirror finishing, and coloring. 1.2.1 Natural Color Whitening During manufacturing, stainless steel may develop a hard gray-black oxide scale due to rolling, edging, welding, or heat exposure. This scale consists primarily of NiCr2O4 and NiF. Traditionally, hydrofluoric acid and nitric acid were used for removal, but this method is costly, environmentally harmful, and dangerous. Today, two main alternatives exist: ⑴ Sandblasting: Micro-glass beads are sprayed onto the surface to remove the oxide layer. ⑵ Chemical Method: A non-contaminant pickling passivation paste is applied at room temperature using inorganic additives. The result is a dull finish, ideal for large or complex products. 1.2.2 Mirror Finishing Techniques such as mechanical polishing, chemical polishing, and electrochemical polishing can achieve mirror-like finishes depending on the product's complexity and user specifications. Each method has its pros and cons. 1.2.3 Coloring Coloring stainless steel enhances its visual appeal and improves wear and corrosion resistance. Various methods include: ⑴ Chemical Oxidation Coloring: Film color is formed via chemical oxidation in a specific solution. ⑵ Electrochemical Oxidation Coloring: Film color develops through electrochemical processes. ⑶ Ion Deposition Oxide Coloring: Workpieces are placed in a vacuum chamber for plating, often resulting in gold tones for watches. ⑷ High-Temperature Oxidation Coloring: A colored oxide film forms in molten salts at specific temperatures. ⑸ Gas Phase Cracking Coloring: A more complex method rarely used industrially. 1.3 Choosing the Right Method The appropriate treatment depends on the product’s structure, material, and desired surface finish. Causes of Rust in Stainless Steel 2.1 Chemical Corrosion 2.1.1 Surface Contamination: Oil, dust, acids, alkalis, and salts can degrade into corrosive agents under certain conditions, reacting with stainless steel components and causing rust. 2.1.2 Surface Scratches: Damage to the protective oxide film weakens stainless steel’s corrosion resistance, leading to rust. 2.1.3 Poor Cleaning: Residual chemicals left after pickling and passivation can corrode the steel directly. 2.2 Electrochemical Corrosion 2.2.1 Carbon Steel Contamination: Contact with carbon steel and corrosive media can create galvanic cells, leading to rust. 2.2.2 Cutting: Adhesion of rust-prone substances like cutting slag and splashes can form galvanic cells. 2.2.3 Heat Treatment: Uneven heating can lead to galvanic cell formation with corrosive media. 2.2.4 Welding: Defects in welds (pores, cracks, etc.) and chemical imbalances can cause electrochemical corrosion. 2.2.5 Material Defects: Uneven composition or surface flaws can facilitate galvanic cell formation. 2.2.6 Passivation Failures: Poor passivation can leave thin or uneven films, making the steel prone to corrosion. 2.2.7 Residual Chemicals: Remaining pickling passivation residues can cause corrosion. Processing Challenges 3.1 Weld Defects: Severe defects require grinding, leaving visible marks that affect aesthetics. 3.2 Surface Inconsistencies: Uneven welds and pickling lead to unsightly surfaces. 3.3 Persistent Scratches: Overall pickling fails to remove scratches and contaminants like carbon steel splashes, leading to rust. 3.4 Uneven Polishing: Manual polishing followed by pickling yields inconsistent results, increasing costs. 3.5 Limited Pickling Capacity: Some black oxide scales resist removal by standard pastes. 3.6 Human Factors: Bumps, drags, and hammers during handling worsen surface issues. 3.7 Equipment Issues: Bends and creases during shaping contribute to rust. 3.8 Raw Material Handling: Rough handling during procurement and storage also leads to rust. Preventive Measures 4.1 Storage, Handling, and Transport 4.1.1 Stainless Steel Storage: Use dedicated racks with wooden or rubber-coated supports to isolate from carbon steel. Store in a clean, accessible location. 4.1.2 Lifting: Use specialized equipment like slings or chucks to avoid scratches. 4.1.3 Transport: Ensure cleanliness and protection during transit. 4.2 Processing 4.2.1 Fixed Processing Areas: Maintain cleanliness and proper isolation. 4.2.2 Cutting: Use shears, saws, or plasma cutters while minimizing damage. 4.2.3 Machining: Protect workpieces during turning and milling. 4.2.4 Forming: Prevent scratches during rolling and bending. 4.2.5 Riveting and Welding: Avoid forceful assembly and contamination. 4.2.6 Welding: Remove debris before welding, use argon arc welding, and apply anti-splash measures. 4.2.7 Multi-Layer Welding: Clean interlayers and control temperature. 4.2.8 Post-Weld Cleaning: Remove slag and ensure smooth transitions. 4.2.9 Correction: Avoid excessive heating; use mechanical tools. 4.2.10 Transport: Use clean tools and avoid direct dragging. 4.3 Surface Treatment 4.3.1 Cleaning and Grinding: Thoroughly remove scratches, splashes, and carbon steel damage. 4.3.2 Polishing: Achieve uniform finishes without over-polishing. 4.3.3 Degreasing: Remove oil, scale, and dust before pickling. 4.3.4 Sandblasting: Select appropriate glass beads and parameters. 4.3.5 Pickling and Passivation: Follow strict protocols. 4.3.6 Rinsing and Drying: Neutralize, rinse, and dry thoroughly. 4.3.7 Protection: Shield treated surfaces from secondary contamination. 4.3.8 Avoid Reprocessing: Once treated, avoid unnecessary modifications. By implementing these strategies, the longevity and appearance of stainless steel products can be greatly improved.

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The invention and use of stainless steel can be traced back to the First World War. At that time, the British guns on the battlefield were always transported back to the rear due to wear and tear. The military production department ordered Brearley to develop high-strength wear-resistant alloy steel, specializing in solving the wear problem of the gun chamber. Brearley and his assistants collected various types of steel and alloy steels produced at home and abroad, conducted performance experiments on various types of machinery, and then selected more suitable steels to make guns. One day, they experimented with an alloy steel containing a large amount of chromium. After the wear resistance test, they found that the alloy was not wear-resistant, indicating that it could not make guns. So they recorded the experimental results and threw it in the corner. . One day a few months later, an assistant rushed to Brearley with a piece of shiny steel and said, "Sir, this is the alloy steel sent by Mr. Mullah I found when I was cleaning the warehouse. You Do you want to experiment to see what special effect it has!" "Okay!" Brearley said happily, looking at the bright and dazzling steel.
The experimental results show that it is a stainless steel that is not afraid of acid, alkali and salt. This stainless steel was invented by Mullah in Germany in 1912. However, Mullah did not know what this stainless steel was used for.
Brierley thought to himself: "This kind of non-wear-resistant but corrosion-resistant steel can't be used to make guns. Can it be used to make tableware?" He went ahead and made a stainless steel fruit knife, fork, spoon, and fruit plate and folding knives.

The stainless steel invented by Brearley was patented in the United Kingdom in 1916 and began to be mass-produced. Since then, stainless steel accidentally discovered from garbage heaps has swept the world, and Henry Brearley is also known as the "father of stainless steel".

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