Medium carbon steel material suitable for manufacturing medium-strength mechanical parts, shaft parts, connectors, quenching and tempering parts, wear-resistant parts, fixtures and jigs, and ordinary mechanical structural parts.
40# is a medium-carbon high-quality carbon structural steel with good strength, hardness, wear resistance, and machinability. After quenching and tempering, quenching, tempering, and other heat treatments, it achieves better overall mechanical properties. Compared to 20# steel, 40# steel has higher strength and hardness, but its plasticity, weldability, and cold forming properties are weaker; Compared to 45# steel, 40# steel has slightly lower strength but offers more balanced processing and toughness. It is commonly used in shaft parts, gears, connecting rods, pins, sleeves, mechanical structural parts, fixtures and jigs, and medium-load metal parts.
By focusing on application scenarios, boundaries of advantages, and non-recommended scenarios, it helps sales, customer service, and quoting staff quickly determine whether the material meets current part requirements.
It has good strength, good hardness, wear resistance superior to low-carbon steel, good machinability, can be strengthened by heat treatment, has relatively low material costs, is suitable for shaft parts and medium-load structural parts, and is suitable for use after quenching and tempering.
Shaft parts, pins, sleeves, connecting rods, gear blanks, sprockets, screws, connectors, clamps and jigs, mechanical brackets, ordinary transmission parts, mold auxiliary parts, hardware parts, medium-load structural parts, and mechanical parts after quenching and tempering.
High-strength heavy-duty parts, high-impact toughness parts, highly corrosive environment parts, long-term exposed outdoor parts, food direct contact parts, medical implant parts, strong welded structural parts, high-elasticity snap-fit parts, high-precision high-speed gears, and metal parts requiring rust-free maintenance.
The following parameters come from product information and material knowledge fields, used for design review, quotation communication, and preliminary judgment before material selection.
| Material positioning | Medium carbon steel material suitable for manufacturing medium-strength mechanical parts, shaft parts, connectors, quenching and tempering parts, wear-resistant parts, fixtures and jigs, and ordinary mechanical structural parts. |
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| Precision performance | 40# suitable for CNC turning, milling, drilling, tapping, grinding, and finishing after heat treatment. CNC machining can achieve better dimensional accuracy; After heat treatment, dimensional accuracy is affected by deformation, and key dimensions usually require grinding or secondary finishing. It is recommended to mark tolerances and machining requirements separately for shaft parts, hole positions, threads, mating surfaces, and wear-resistant surfaces. |
| Dimensional tolerances | Conventional dimensional tolerances for CNC machining 40# can be referenced as ± 0.02mm to ±0.10mm, while ordinary structural parts can be evaluated at ± 0.10mm to ±0.20mm; If parts require quenching and tempering, quenching, or surface quenching, dimensional changes or deformation may occur after heat treatment. It is recommended to heat treat key dimensions before finishing. These values represent the standard reference range and do not guarantee absolute tolerances for all structures. Actual confirmation must be based on process, dimensions, structure, heat treatment, and post-processing. |
| Minimum Wall Thickness | The wall thickness of ordinary CNC machined structures is recommended to be no less than 0.8mm-1.0mm. Small non-stressed areas can be further optimized according to structure and processing methods, but it is not recommended to make large areas that are too thin. Areas requiring heat treatment, tapping, load-bearing, wear resistance, or assembly should be appropriately thickened to avoid processing deformation, heat treatment deformation, cracking, or insufficient strength. |
| Recommended wall thickness | For ordinary structural parts, 1.5mm-3.0mm is recommended; for shaft parts, mounting seats, threaded connection points, fixtures and jigs, wear-resistant surfaces, and load-bearing positions, it is recommended to be above 2.0mm; for parts requiring heat treatment, increase machining allowance based on deformation risk and reserve grinding or finishing allowances at key dimensional positions. |
| Minimum aperture | CNC machining can achieve smaller hole diameters, but controlling deep small holes and hole position accuracy after heat treatment is more challenging. For general designs, the recommended aperture is no less than 1.0mm. For precision holes, threaded holes, positioning holes, pin holes, and fitting holes, it is recommended to reserve machining allowance. If necessary, drill, ream, boring, or grinding should be performed after heat treatment. |
| Assembly clearance | For precision metal assembly, one side can be reserved at 0.02mm-0.10mm according to fitting requirements; for ordinary plug-in and assembly, it is recommended to reserve 0.10mm-0.30mm per side. If the part requires heat treatment, blackening, electroplating, powder spraying, painting, or rust prevention, combine heat treatment deformation and surface treatment thickness to increase assembly allowance, avoiding over-tight assembly or scratching of the surface layer. |
| Detailed performance | 40# steel is suitable for machining holes, grooves, steps, chamfers, threads, shaft shoulders, positioning surfaces, keyways, wear-resistant surfaces, and ordinary mechanical structural details. Under CNC machining, detail performance is good, and after heat treatment, key surface quality can be improved through grinding. Small text, logos, and markings are recommended to be achieved through laser marking, engraving, stamping, or etching. |
| Surface effect | The original surface of 40# steel is usually gray-black or silver-gray metallic, and may have oxide scale, knife marks, scratches, or rust spots. After grinding and polishing, a relatively smooth metal surface is obtained; after blackening, a black rust-resistant appearance is achieved; after galvanizing or electroplating, a silver-white or glossy metallic effect can be achieved; after painting or powder spraying, multiple colors and better protection can be achieved. If long-term appearance stability is desired, reliable anti-rust treatment or switching to stainless steel is recommended. |
Based on material characteristics and suitable product ranges, customer needs are broken down into easier application directions to determine.
Shaft parts, pins, sleeves, connecting rods, gear blanks, sprockets, screws, connectors, clamps and jigs, mechanical brackets, ordinary transmission parts, mold auxiliary parts, hardware parts, medium-load structural parts, and mechanical parts after quenching and tempering.
Medium carbon steel material suitable for manufacturing medium-strength mechanical parts, shaft parts, connectors, quenching and tempering parts, wear-resistant parts, fixtures and jigs, and ordinary mechanical structural parts.
40# is a medium-carbon steel, with strength, hardness, and wear resistance better than low-carbon steels like 10# and 20#, making it suitable for manufacturing mechanical parts that require certain load-bearing capacity and wear resistance. It can improve performance through heat treatments such as quenching and tempering, quenching, and tempering, but its weldability and plasticity are inferior to low-carbon steel. As ordinary carbon steel, it is prone to rust in exposed environments and usually requires surface protection such as blackening, electroplating, painting, phosphating, and anti-rust oil.
Based on wall thickness, hole position, assembly clearance, dimensional tolerances, and material usage risks, determine in advance whether the part structure is suitable for 40#.
Post-processing of 40# affects appearance, dimensions, hole position, assembly clearance, and usage validation results, and should be explained in advance during quotation, DFM review, and sample confirmation stages.
Post-processing should focus on appearance display, dimensional fitting, connection assembly, and testing verification. Parts involving assembly positions need to be reserved in advance for machining, coating, and trial assembly allowances.
For common issues such as threads, snaps, strength, temperature resistance, and weather resistance, identify in advance whether the material needs to be replaced or if another processing method should be used.
When customer demand exceeds 40# material boundaries, it is necessary to combine strength, temperature resistance, toughness, long-term stability, and mass production goals to promptly recommend alternative materials or processing technologies.
If higher strength and hardness are required, 45# steel, 40Cr, 42CrMo, or alloy steel can be chosen; If better weldability and formability are required, 20# steel or 10# steel can be chosen; If better corrosion resistance is required, 304 stainless steel, 316 stainless steel, or surface galvanized treatment can be chosen; If you need a lighter weight, you can choose the 6061 Aluminum Alloy or 7075; If a highly elastic structure is needed, 65Mn, 301 stainless steel, or spring steel can be chosen.
If customers require higher strength, hardness, and wear resistance than low-carbon steel, while also wanting to control material costs,40# is a suitable choice. If customers need to use it outdoors for long periods in humid or corrosive environments, it is not recommended to use exposed 40# steel directly; stainless steel should be chosen or reliable surface protection should be added. If customers require high strength under heavy load or high fatigue life, materials such as 45# steel, 40Cr, and 42CrMo should be further evaluated.
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