The method of using magnets to detect stainless steel has great limitations

1. The magnetism of stainless steel is directly related to its metallographic structure ‌Austenitic stainless steel (such as 304, 316)‌ Theoretically, it is non-magnetic, but in actual production, it may produce weak magnetism due to cold processing (such as stamping, bending) or composition deviation. For example, 304 stainless steel may be magnetic after processing due to the partial conversion of austenite to martensite, but this does not affect its corrosion resistance. Martensitic/ferritic stainless steel (such as 430, 420)‌ It is magnetic in itself and is often used for knives, fasteners, etc., but this type of material still belongs to the category of stainless steel. For example, a 430 stainless steel frying pan can be adsorbed by a magnet, but it is indeed a qualified stainless steel product. 2. Three major misunderstandings of magnet detection ‌False negative risk‌ Non-stainless steel materials such as zinc alloys and copper alloys are also non-magnetic and can be easily misjudged as "high-quality stainless steel". False positive risk‌ Some inferior "stainless iron" (recycled steel demagnetization treatment) may be non-magnetic, but the anti-rust performance is extremely poor. Ignore performance differences ‌ Corrosion resistance (such as 316>304>430) has no direct correlation with magnetism. Austenitic stainless steel is still better than magnetic ferritic stainless steel even with weak magnetism. 3. More scientific identification method ‌Professional testing ‌ Analyze the chromium and nickel content through a spectrometer (304 needs to contain 18% chromium and 8% nickel). Observe the mark ‌ Regular products will be marked with the material (such as SUS304), avoid buying unmarked products. Corrosion test ‌ Observe the rust condition by salt spray test or soaking in acidic solution (such as lemon juice).

About stainless steel forging

1 Preparation before forging (1) Billet selection Select appropriate billets according to the final product shape and performance requirements of stainless steel. Billets can be bars, plates or ingots. For parts with complex shapes, ingots may need to be selected for forging to ensure the uniformity of the material. At the same time, the size of the billet should be considered, and sufficient processing allowance should be left. The allowance generally ranges from a few millimeters to more than ten millimeters, depending on the size and precision requirements of the parts. (2) Heating equipment Stainless steel forging requires appropriate heating equipment. Commonly used ones are resistance furnaces and gas furnaces. Resistance furnaces have accurate heating temperature control and high thermal efficiency, which can make the stainless steel billet heated evenly. Gas furnaces have fast heating speed, but the temperature control is relatively complicated. Control of heating temperature is crucial for stainless steel forging, because different types of stainless steel have different starting and ending forging temperature ranges. For example, the starting forging temperature of austenitic stainless steel is generally around 1150-1200℃, and the ending forging temperature is not less than 900℃. 2 Forging process (1) Forging methods It includes free forging and die forging. Free forging is highly flexible and is suitable for small-batch production of single pieces or large parts with simple shapes. Forging workers gradually shape the blank by operating forging tools such as air hammers and steam-air hammers. Die forging uses a die to shape the blank, which has high production efficiency and is suitable for mass production of parts with complex shapes and high dimensional accuracy requirements. For example, stainless steel connecting rods for automobile engines are usually produced by die forging. (2) Selection of forging ratio The forging ratio is an indicator of the degr

Common specifications of PPGI

Parameter Range Typical value Substrate thickness 0.3~1.2mm 0.5mm Alu-zinc coating weight 60~150g/m² (double-sided) 90g/m² Coating total thickness 20~35μm (double-sided) 25μm Board width 600~1250mm 1000mm Roll weight 3~10 tons 5 tons 2. Application areas Architecture: roofs, walls, curtain walls (such as airports and gymnasiums). Home appliances: refrigerator panels, air conditioner shells, oven liner. Transportation: compartment partitions, container shells. Furniture: office partitions, cabinet door panels. 3. Differences from PPGI and PPGL Type Substrate Features Main uses PPGI Galvanized sheet (GI) Low cost, weak corrosion resistance Ordinary buildings, home appliances PPGL Alu-zinc (GL) Excellent corrosion resistance/heat resistance, long life High-end buildings, industrial environments PPAL Aluminum-coated sheet (AL) High temperature resistance (up to 650°C), good reflectivity Chimneys, ovens 4. Purchase considerations Coating type: PVDF for outdoor use, polyester for indoor use. Coating weight: ≥90g/m² is recommended for coastal/industrial areas. Environmental protection standards: Confirm that there is no chromium treatment (RoHS compliant). Surface texture: Embossing, matte and other processes affect the appearance and stain resistance. PPGI has dual protection through substrate + coating, combining functionality and aesthetics, and is an ideal material to replace traditional spraying

PPGI steel (aluminum-zinc coated steel sheet)

PPGI (Pre-Painted Galvalume Steel Coil/Sheet) is a pre-painted aluminum-zinc coated steel sheet, which combines the corrosion resistance of the aluminum-zinc coated substrate and the decorative/protective performance of the color coating. It is widely used in construction, home appliances and other fields. The following are its key knowledge points: 1. Substrate material: Galvalume (aluminum-zinc coated) PPGI's metal substrate is aluminum-zinc coated steel sheet (55% aluminum, 43.4% zinc, 1.6% silicon), and its performance is better than ordinary galvanized sheet (GI): Corrosion resistance: The corrosion resistance of aluminum-zinc coated steel is 2 to 6 times that of galvanized sheet (especially in high temperature and high humidity environments). Heat resistance: It can withstand high temperatures of 300°C for a long time (galvanized sheet is only about 200°C). Reflectivity: The aluminum layer reflects ultraviolet rays and delays the aging of the coating. 2. Coating structure (five-layer composite) Primer (epoxy resin/polyester) 5~10μm Topcoat (PVDF/polyester/SMP) 15~25μm Chemical conversion layer (chromate/chrome-free) 0.1~0.5μm Alu-zinc coating (Galvalume) 20~150g/m² Rolled steel substrate 0.3~1.2mm Coating type comparison: PVDF (fluorocarbon): super weather-resistant (more than 20 years), UV-resistant, used for high-end building curtain walls. Polyester (PE): low cost, rich colors, suitable for home appliances and interior decoration. SMP (silicon-modified polyester): weather resistance is between PVDF and PE. 3. Core performance indicators Corrosion resistance: Salt spray test (alu-zinc coating substrate): ≥1000 hours (red rust appearance time). The scratch expansion width of the color-coated plate is ≤2mm (ASTM D1654). Adhesion: Cross-hatch test level 0 (no peeling). Hardness: Pencil hardness ≥ H (PVDF coating can reach 2H). Temperature resistance: -40°C~120°C without cracking.

Innovation and Trends of Alu-Zn Plating

Environmentally friendly passivation technology Chromium-free passivation (such as silane treatment) meets the RoHS directive. Composite coating Alu-Zn + PVDF coating (such as PPGL), weather resistance is improved to 30 years. High reflective coating Specialized for solar backplane (reflectivity ≥ 80%, improve power generation efficiency). Selection and use suggestions Coating selection Coastal/industrial areas: AZ150 or above (high corrosion resistance). Dry inland: AZ75-AZ100 (cost saving). Processing Notes After cutting, the edge needs to be sprayed with zinc-aluminum repair paint. Avoid direct contact with copper and stainless steel (galvanic corrosion). Frequently asked questions Q1: Can aluminum-zinc plating replace stainless steel? Some scenarios can: such as building roofs (the cost is only 1/3 of stainless steel), but chemical equipment still needs stainless steel. Q2: How long is the service life of galvalume? Ordinary environment: 25-30 years (coating AZ150). Industrial area: 15-20 years (need to check the incision regularly). Q3: How to identify inferior galvanized steel sheets? Surface: uniform matte silver-gray, no zinc flower (galvanized steel sheets have zinc flower patterns). Measure the coating: XRF spectrometer detects the aluminum-zinc ratio.

PPGI market trends and innovations

Environmentally friendly PPGI Chromium-free passivation (such as titanium zirconium treatment), water-based coatings (VOC emissions reduced by 50%). Functional coatings Self-cleaning coatings (photocatalytic TiO₂), antibacterial coatings (silver ions). Smart PPGI Thermostatic coatings (change color with temperature), photovoltaic integration (solar panel substrate). Selection suggestions Outdoor buildings: PVDF coatings (weather resistance > 20 years) are preferred. Cost sensitive: Polyester (PE) coating, taking into account both performance and price. Harsh environment: Aluminum-zinc substrate (PPGL) + thick coating (≥25μm). Frequently Asked Questions Q1: How to choose PPGI and stainless steel? PPGI: Suitable for scenes with limited budget and color requirements (such as roofs). Stainless steel: Stronger corrosion resistance, but high cost and single color. Q2: How long is the life of PPGI? Normal environment: PE coating is about 10-15 years, PVDF coating is 15-25 years. Industrial area/coastal area: life is shortened by 30%, regular maintenance is required. Q3: How to identify high-quality PPGI? Check the zinc layer: use a magnetic thickness gauge to detect (double-sided zinc layer ≥ 60g/m²). Measure adhesion: tape tear test (no coating falling off).