When mechanical design engineers select raw materials, their core requirements always focus on four points: high strength, outstanding toughness, easy CNC processing and controllable procurement cost. As a result, S45C (JIS G4051), a medium-carbon non-alloy structural steel with balanced comprehensive performance, ranks among the most widely used carbon steels globally. Most online technical articles only list basic parameters while ignoring practical production pitfalls and wrong material selection on-site. Combined with actual mass production data from machining workshops and metallurgical principles, this guide sorts out S45C from definition, metallographic structure, optimized heat treatment, CNC processing technology, steel grade comparison and application limits, helping engineers cut rejection rate and control production cost.
1 Basic Definition & Global Equivalent Steel Grades
S45C is classified as premium medium-carbon non-alloy structural steel with three core features: average carbon content at 0.45%, free of expensive Cr/Ni alloy elements and compatible with full-range heat treatment including normalizing, quenching & tempering, induction surface hardening. The 0.45% carbon content hits the perfect balance: lower carbon leads to insufficient hardening capacity after heat treatment, while higher carbon reduces toughness and machinability, which distinguishes it from low-carbon welding steel and high-carbon wear-resistant steel.
| Standard System | Grade | Main Application |
| JIS (Japan) | S45C | Load-bearing mechanical parts, mold base blank |
| AISI (USA) | 1045 | General mechanical shafts, equipment accessories |
| DIN (Europe) | C45/CK45(1.0503) | Quenched structural components for construction machinery |
Engineer Tip: Equivalent grade does not mean identical material performance. Differences in deoxidation process and sulfur content from different steel mills change cutting performance and hardenability significantly. Request official mill test certificate for components under fatigue load.
2 Chemical Composition & Element Functions
| Element | Content Range | Practical Function |
| C | 0.42~0.48% | Core element to decide max hardness after quenching; 0.45% balances surface wear resistance and core toughness |
| Mn | 0.60~0.90% | Refine grain structure, improve strength and hardenability, lower cracking risk during quenching |
| Si | 0.15~0.35% | Deoxidize molten steel during smelting and slightly boost tensile strength |
| P | ≤0.030% | Harmful impurity; excessive phosphorus causes cold brittleness of finished parts |
| S | ≤0.035% | Excess sulfur damages toughness and reduces surface finish during precision machining |
Industry field data: Unqualified cheap S45C has excessive sulfur and phosphorus up to 0.07%, which raises the rejection rate of quenched parts by 38% under identical heat treatment procedures. It is the hidden cause of frequent defective products after purchasing low-cost raw materials for many processing factories.
3 Metallographic Structure & Mechanical Property Changes
The performance fluctuation of S45C originates from metallographic transformation caused by different heat treatments, two mainstream structural states are listed below:
3.1 Normalized State: Ferrite + Pearlite Mixed Structure
Soft ferrite improves material toughness and cutting property; layered pearlite guarantees basic mechanical strength. It is the optimum raw material status for CNC roughing and finishing, with easy chip breaking and low tool loss. Over 80% of blank machining chooses normalized S45C as incoming material.
3.2 Quenched & Tempered State: Tempered Martensite Structure
Quenching generates high-hardness martensite, and high-temperature tempering eliminates internal quenching stress to balance high tensile strength and fatigue resistance. One obvious shortcoming is limited hardenability without alloy additives: components thicker than 60mm cannot be fully hardened to core, with residual soft pearlite inside, so full-section quenching is not recommended for thick-wall heavy-load parts.
4 Mechanical Properties under Different Treatment Status
| Treatment Status | Tensile Strength(MPa) | Yield Strength(MPa) | Elongation(%) | Hardness | Applicable Working Condition |
| NormalizedBlank | 570~650 | 300~350 | 16~20 | 160~220HB | Regular CNC machining, non-load-bearing base plate |
| Standard Quenched & Tempered | 650~800 | 400~550 | 14~18 | 200~260HB | General transmission shafts, medium-size gears |
| Enhanced Quenched & Tempered | 800~950 | 550~650 | 10~14 | 280~350HB | Heavy-load pins, precision transmission accessories |
| Induction Surface Hardening | Same as tempered base | - | - | Surface:50~55HRC | Shaft journal, gear tooth surface requiring local wear resistance |
Basic physical parameters: Density:7.85g/cm³, Elastic Modulus:205GPa (rigidity unchanged by heat treatment), Linear Thermal Expansion Coefficient:11.5µm/m·℃.
5 Practical Heat Treatment Process & Avoidable Defects
All processes optimized based on mass production experience instead of theoretical textbook parameters:
Normalizing: Heat to 840~860℃ then air cooling Ideal for forged or hot rolled blanks with coarse grains to unify hardness, reduce vibration and dimensional deformation in CNC processing. Pre-normalization on welding blanks cuts cold cracking risk by 75%.
Quenching & Tempering: Soak at 830℃ then water/oil quenching + temper at 550~650℃ Water quenching for workpieces below 25mm for higher hardness; oil quenching for parts over 30mm to prevent cracking. Higher tempering temperature brings better toughness but lower hardness.
Stress-relief Annealing: 810℃ furnace cooling slowly Applied to blanks with uneven hardness or large deformation after rough machining to eliminate rolling residual stress and stabilize finished size.
Local Induction Hardening Only harden friction surface while core keeps original tempered toughness, the most cost-effective reinforcement solution for gears and transmission shafts.
6 DFM Tips for CNC Machining & Welding
6.1 CNC Machining Suggestion
Normalized blank: Excellent machinability, adopt TiAlN coated cutting tools with medium feed speed for smooth chip removal and minimum processing cost in mass production.
Tempered workpiece over 30HRC: Reduce cutting speed by 20% and use full cemented carbide tools; adopt peck drilling for deep hole machining to remove scraps timely.
6.2 Welding Specification Warning
S45C owns relatively high carbon equivalent with high cold cracking risk, avoid welding as much as possible. If welding is inevitable: preheat base material to 150~250℃, apply low-hydrogen welding electrode and complete stress relief tempering at 600℃ after welding. Replace with S20C low carbon steel for bulk welded structural parts.
7 Material Selection Comparison Between S45C and Common Structural Steel
| Grade | Carbon Content | Overall Strength | Machinability | Weldability | Preferred Application |
| S45C | 0.45% | Medium-high | Excellent | Fair | Mold base, transmission shaft, medium-load gear, high-strength fastener |
| S20C(1020) | 0.20% | Low | Outstanding | Perfect | Welding frame, sheet metal welded components |
| S50C | 0.50% | High | Average | Poor | Light-load wear-resistant small parts, simple wear bushing |
| SCM440(4140) | 0.40% | Very high | Average | Difficult | Thick-wall high-fatigue safety components, heavy-duty transmission shaft |
Core Selection Logic
Full welded construction → S20C; Cost-effective load-bearing machined parts → S45C; Thick-wall heavy fatigue components → SCM440.
8 Applicable & Forbidden Application Scenarios of S45C
Available Application
Automotive parts: Connecting rod, half shaft, suspension support shaft;
General machinery: Main shaft, sprocket, medium-size spur gear;
Mold industry: Plastic injection mold base, backing plate, fixture base;
Standard fasteners: Grade 8.8 high-strength bolt, positioning pin.
Unavailable Scenarios
Humid & corrosive environment: Prone to rust without anti-corrosion coating, switch to S136 stainless steel for food or acid-exposed working condition;
Severe high-abrasion workpiece: Adopt P20 pre-hardened mold steel or tool steel for mass-production mold cavity;
Heavy-duty high-impact safety parts: Replace with SCM440 alloy structural steel for large construction machinery components.
9 Our Customized S45C Steel Service
We keep full-size S45C round bar, hot rolled plate and six-side precision ground blank in stock all year round. According to customer drawings, we provide sawing, milling and pre-drilling pre-processing service, so clients can directly start cnc machining after receiving goods. All batches come with original mill certificate, and our technical team offers free optimization on heat treatment and processing parameters.
FAQ
Q1: Can secondary precision machining be conducted after S45C quenched & tempered?
A: Regular tempered hardness ranges from 22~32HRC, which can be normally milled with cemented carbide tools; special enhanced tempered steel over 35HRC requires dedicated wear-resistant cutters and optimized cutting parameters.
Q2: Can S45C be directly used for outdoor equipment frame?
A: Available with black oxide or galvanized anti-rust treatment; choose low-carbon stainless steel for long-term outdoor application without anti-corrosion protection.
Q3: How to select between cold drawn and hot rolled S45C for cost control?
A: Choose cold drawn round bar for mass production of small-size shafts to save rough machining cost; pick hot rolled blank for large thick base to cut raw material expense.
Q4: Is S45C suitable for plastic injection mold cavity?
A: Only available for small-batch trial sample molds. It lacks wear and rust resistance for mass production, P20 pre-hardened mold steel is the better option for formal mold cavity.