I.clear 304 stainless steel core standards: chemical composition and metallurgical process
304 stainless steel “high quality” is first reflected in the precise control of its chemical composition. According to international standards (such as ASTM A240/A240M), the typical composition of 304 stainless steel is as follows:
Chromium (Cr) content: 18%~20%
Nickel (Ni) content: 8%~10.5%
Carbon (C) content: ≤0.08%
Other elements: manganese (Mn) ≤2%, silicon (Si) ≤1%, phosphorus (P) ≤0.045%, sulfur (S) ≤0.03%
Key points Analysis:
1, insufficient nickel content is a common trap: nickel is the core element that determines the corrosion resistance and toughness of 304 stainless steel. Some poor quality plates may reduce the cost by reducing the nickel content (such as only 6%~7%), resulting in materials that are prone to rust in a humid or acidic environment. When choosing and selecting need to ask the supplier to provide third-party testing of the spectral analysis report, and focus on checking the proportion of nickel and chromium.
2, carbon content and corrosion resistance: the lower the carbon content (such as 304L, carbon content ≤ 0.03%), the stronger the resistance to intergranular corrosion. If the product is used for welding processing or high-temperature environment, it is recommended to prioritize the low-carbon version.
3, the impact of metallurgical process: high-quality 304 stainless steel need to control impurities through the AOD (argon oxygen decarburization) refining process to ensure uniform composition. Small steel mills may use backward smelting technology, resulting in the presence of pores or inclusions within the material.
II. the surface quality and processing technology: from finish to tolerance control.
The surface treatment of stainless steel plate directly affects its aesthetics, cleanliness and corrosion resistance. Plates with different processing techniques are suitable for different scenarios:
Purchasing Precautions:
Checking surface defects: high-quality plates should be free of scratches, pits, roller marks, color difference. Available strong light lateral irradiation, observe whether the uniform reflection.
Thickness tolerance control: the actual thickness of 304 stainless steel plate may deviate from the nominal value. For example, the nominal 1.0mm plate, if the ordinary tolerance standard (± 0.1mm), the actual may be between 0.9 ~ 1.1mm; and precision tolerance (such as ± 0.05mm) of the plate is more suitable for processing parts with strict size requirements.
Edge processing: the shear edge should be flat and burr-free, and the end face of the rolled plate needs to be neat and crack-free.

III. mechanical properties and corrosion resistance: laboratory testing and practical verification.

In addition to the composition of the standard, the mechanical properties and corrosion resistance of 304 stainless steel need to be verified through practical testing:

1, mechanical properties test
Tensile Strength (Tensile Strength): ≥ 515 MPa
Yield Strength (Yield Strength): ≥ 205 MPa
Extension ( Elongation: ≥40% (≥35% when the thickness is ≤0.5mm)
Test Method: Intercept the sample and send it to the third-party laboratory, and obtain the data through the universal material testing machine.
2, corrosion resistance verification
salt spray test: quality 304 plate in 5% neutral salt spray environment should be at least 72 hours without red rust.
Actual environment simulation: if used in food processing equipment, citric acid or sodium chloride solution can be used to soak for 48 hours to observe whether the surface discoloration.
Intergranular corrosion test: Through the sulfuric acid - copper sulfate boiling method (GB/T 4334) to detect corrosion resistance after welding or heat treatment.

IV. Price and cost-effectiveness: avoid the low price trap
The price of 304 stainless steel plate is greatly affected by the fluctuation of nickel price (nickel accounts for about 50% of the cost), but too low a price is often hidden risks:
Reasonable price range reference (in 2023, for example):
1.0mm thickness cold rolled plate: about 18~25 yuan/kg (brand different differences)
Precision Embossed Plate: mark up of 30%~50%
Sources of cost differences:
Proportion of scrap: large mills mostly use virgin material, small mills may be mixed with recycled material.
Surface treatment process: BA surface costs about 15% more than 2B surface.
Transportation and storage: The stock environment of regular agents can avoid moisture or contamination of the plate.
Suggested Strategies:
Request samples for performance testing before bulk purchase.
Compare the comprehensive offer of 3 to 5 suppliers, and give priority to businesses that provide testing services.
V.practical testing techniques: quickly identify poor quality plate.
Even without professional equipment, you can also use the following methods to initially determine the quality of 304 stainless steel:
1. magnetic test:
304 stainless steel is austenitic structure, under normal circumstances non-magnetic (or weakly magnetic). If the plate can be strongly adsorbed by the magnet, it may be counterfeit 200 series stainless steel (such as 201, 202).
2.potion detection:
use stainless steel identification fluid (such as Mn, Ni detection fluid):
drop on the potion, if the rapid red (high manganese and low nickel), it is 201 stainless steel;
304 stainless steel is usually reddish or colorless (according to the potion formula is different).
3.spark observation:
When grinding with a wheel, 304 stainless steel spark is straight and short, spark fork less; and 200 series stainless steel spark is more dense, fork more.
In summary, selecting high quality 304 stainless steel plate is a job that requires a combination of technology and experience. By strictly controlling the composition, process, testing and supply chain, users can effectively avoid risks and ensure that the material performance meets the needs of long-term use. It is recommended to specify the quality terms in the procurement contract and keep samples for traceability in case of dispute. In the pursuit of cost optimization at the same time, do not ignore the essential performance of the material - after all, a piece of high-quality 304 stainless steel plate, may be ten years of rust-free guarantee, and the hidden loss of poor-quality plate may far exceed the price difference in procurement.

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