Knowledge

Let's take a detailed look at the material selection principles for forged gear blanks and the typical application scenarios of different materials. Forging, as one of the main forming methods for gear blanks, is crucial in material selection, which directly affects the final performance, lifespan, processing cost, and application feasibility of the gear.

I. Main Considerations in Selecting Materials for Forged Gear Blanks

 

1. Performance Requirements:

-Strength and Hardness: Core requirements to resist bending fatigue fracture at the tooth root and surface crushing deformation. Must meet the gear's load-carrying capacity requirements (contact strength, bending strength).

 - Wear Resistance: Resistance to pitting, wear, and scuffing on the tooth surface. Especially critical for pitting near the pitch line.

-Toughness: Absorb impact loads and prevent sudden tooth breakage.

- Fatigue Limit: Resistance to fatigue failure under alternating loads.

- Heat Treatment Performance: Whether the material can be easily heat-treated through quenching, tempering, carburizing, nitriding, etc., to achieve the desired properties (hard surface, tough core). This is a key consideration.

- Hardenability: Very important for small gears requiring overall hardening or controlling the depth of hardening in the tooth area of gears that only need local hardening.

 

 

2. Operating Conditions:

- Load Magnitude and Nature: Static, dynamic, or impact load? Light, medium, heavy, or super-heavy load?

- Speed: High-speed operation requires consideration of dynamic balance, centrifugal force, and overheating risk.

- Temperature: Abnormally high or low working temperature (requiring heat-resistant or high-toughness materials)?

- Environment: Exposure to corrosive media, moisture, dust, vacuum? Requires corrosion resistance or special surface treatment.

- Lubrication Conditions: Grease lubrication, forced oil spray lubrication, splash lubrication, or even dry friction?

- Expected Life and Reliability Requirements: Degree of requirements for wear and fatigue life.

 

3. Manufacturability:

- Forgeability: The material should have good hot plastic deformation ability, be easy to forge into shape, reduce forging defects, and minimize mold wear.

- Machinability: The machinability before and after heat treatment for turning, milling, hobbing, shaping, and grinding.

- Weldability: Is gear welding or subsequent welding assembly required?

- Heat Treatment Sensitivity: Degree of quenching deformation? Tendency to crack? Sensitivity to decarburization?

 

4. Cost Effectiveness:

- Material Cost: Raw material prices vary significantly.

- Processing Cost: Including forging, heat treatment, and machining costs. Cheaper materials may lead to more complex processes or shorter lifespans.

- Heat Treatment Cost: Carburizing and quenching is more expensive than normalizing, and vacuum heat treatment is more expensive than air furnace treatment.

- Supply Chain Stability: Ease of material procurement and delivery cycle.

- Total Cost of Ownership (TCO): Comprehensive consideration of procurement cost, manufacturing cost, usage cost (wear, maintenance, downtime), and lifespan.

5. Size and Design Factors:

- Gear Size: Large gears are limited by blank specifications and the quenching capacity of heat treatment equipment.

- Structural Complexity: Does it have an attached shaft or spoke structure? Different considerations for materials and heat treatment.

 

Key Points in the Material Selection Process

1. Clearly define the gear's function, operating conditions, load spectrum, and key performance indicators.

2. Preliminarily screen out several types of materials that meet the basic performance requirements based on the requirements (such as high surface hardness? High core toughness? Corrosion resistance?). 。

3. Evaluate manufacturing feasibility and cost: Is it easy to forge, heat treat and process? Is the cost acceptable?

4. Final trade-off: Select the combination with the best performance, process and cost. Alloy structural steel is usually the most common choice.

 

 

 

II. Common Forging Gear Materials and Their Typical Application Scenarios

 

The following are the major categories of materials commonly used for forging gears, along with their typical specifications and application scenarios:

 

1. Medium carbon steel and medium carbon alloy steel - Quenched and Tempered Steels:

Representative grades:

- Domestic: 45 (55), 40Cr (50Cr), 40MnB (45MnB), 42CrMo (35CrMo, 40CrMo, 50CrMo), 40CrNiMo, 34CrNiMo6 (similar to German standard), 30CrMnTi (more focused on carburizing)

- American standard: 1045 (1050), 4140 (4340), 4150, 4340, 4330V, 300M

- German standard: C45, 42CrMo4, 34CrNiMo6, 30CrNiMo8

- Japanese standard: S45C (SCM440), SNCM439, SCM420 (mostly used for carburizing)

- Heat treatment: Quenching + high-temperature tempering (quenching and tempering treatment). Obtain sorbite structure.

- Core performance characteristics:

- Excellent overall mechanical properties (good matching of strength, hardness, plasticity and toughness).

- Core and surface hardness are basically the same, usually in the range of HB250-HB350.

- Good process performance (forging, cutting, welding).

- Typical application scenarios:

- General gears with moderate load, medium speed and no severe impact, where the hardness of the tooth surface is not highly required.

- Medium and low-speed transmission in machine tools, reducers, general mechanical power transmission, agricultural machinery, construction machinery, etc.

- Usually operate under grease or splash lubrication conditions.

- Common choice for large gears (due to the good hardenability of alloy structural steel).

- Advantages: Relatively low cost (especially carbon steel), good processability, stable and reliable performance.

- Disadvantages: Low tooth surface hardness, inferior wear resistance and contact fatigue strength compared to high-hardness surface gears.

 

2. Low carbon alloy steel (carburizing steel/surface hardening steel) (Low-Carbon Alloy Steels - Case Hardening Steels):

- Representative grades:

- Domestic: 20Cr, 20CrMnTi (18CrMnTi), 20CrMnMo, 20CrMo (22CrMo), 20CrNiMo (18CrNiMo7-6), 20Cr2Ni4, 17CrNiMo6 (German standard)

- American standard: 4120 (4320, 4820), 8620, 8822, 9310

- German standard: 20MnCr5 (17Cr3), 16MnCr5, 18CrNiMo7-6, 15CrNi6

- Japanese standard: SCr420 (SCM415), SCM420H, SNCM220

- Heat treatment: Carburizing + quenching + low-temperature tempering (some also use carbonitriding or high-pressure gas carburizing). Form a high-carbon martensitic hardened surface and a low-carbon martensitic/sorbite tough core structure.

- Core performance characteristics:

- Extremely high tooth surface hardness (HRC58-63), excellent wear resistance and pitting resistance.

- Core hardness is adjustable (usually HRC30-45), good toughness and high bending strength.

- Strong tooth load-bearing capacity.

- Typical application scenarios: § For applications with high loads, high speeds, and high precision requirements.

- For automotive transmissions, drive axles (differentials, main reducers), industrial gearboxes (critical grades), wind turbine gearboxes, aviation gears, precision instruments, and situations demanding high reliability and long service life.

- For scenarios with impact loads and cyclic stresses.

- Widely used in gear finishing processes (such as gear grinding) to achieve higher precision and surface finish.

- Advantages: Excellent contact fatigue strength and bending fatigue strength, good wear resistance, and extremely high load-carrying capacity, making it highly suitable for gears subjected to significant point and line contact stresses and bending stresses.

- Disadvantages: Complex heat treatment process (carburizing and quenching), high process control requirements, high cost (both material and heat treatment), significant deformation (requiring subsequent finishing), and relatively lower core toughness compared to quenched and tempered steels.

 

3. Nitriding Steels (Nitriding Steels):

- Representative grades:

- Commonly used: 38CrMoAlA (41CrAlMo7) - Specifically designed for nitriding.

- Also used: 42CrMo (34CrMo4), 40CrNiMo, 34CrNiMo6, and other high-alloy steels with good hardenability (can also be nitrided for better performance).

-Heat treatment: Quenching and tempering (quenching + high-temperature tempering) + surface nitriding (gas/ion nitriding). Forms a high-hardness, wear-resistant, and corrosion-resistant nitrided layer.

-Core performance characteristics:

- High surface hardness and wear resistance (HV1000 and above), excellent red hardness (maintains hardness at 500°C).

- Minimal heat treatment distortion (nitriding temperature is low, approximately 500-560°C).

- Good anti-seizing properties and certain corrosion resistance.

- Significantly improved fatigue strength.

- Typical application scenarios:

- Precision gears where the tooth surface requires extremely high hardness, wear resistance, and fatigue strength, but where deformation must be strictly controlled or where post-heat treatment finishing is difficult.

- Gears operating at higher temperatures (<500-550°C) or in conditions with no or poor lubrication (due to the good anti-seizing properties of the nitrided layer).

 

-  Thin-walled and slender shaft gears that cannot tolerate significant deformation.

 

- Gears in aerospace, precision machine tool spindles, turbocharger gears, high-speed and heavy-duty gears in ships and internal combustion engines, and gears requiring both excellent wear resistance and corrosion resistance.

- Advantages: Ultra-high hardness surface layer, excellent wear and corrosion resistance, minimal distortion, and good high-temperature performance.

- Disadvantages: Thin hardened layer (typically < 0.7mm), not suitable for applications with excessive impact or high contact stresses (risk of deep layer spalling), long nitriding cycle (especially for deep nitrided layers or gas nitriding), and slightly poorer processability for dedicated steels like 38CrMoAlA (such as forgeability, need to eliminate white spots before quenching and tempering).

 

4. Cast Irons (Cast Irons) - Rarely used for forging blanks, but occasionally seen in specific cases

- Characteristics: Lowest cost, good casting properties, excellent vibration damping (reduces noise), and easy to cast complex shapes.

- Application limitations: Rarely used for forging blanks. Mainly used for extremely large or complexly shaped parts that are difficult to forge, typically cast directly into gear form. Malleable cast iron gets its name from the annealing process and is not used for forging blanks.

- Forging and casting interface applications (such as flywheel rings): Occasionally, steel is forged into a ring-shaped blank and then bent into a ring through high-frequency or medium-frequency induction heating (similar to roll forming), but this is very close to profile forming rather than traditional die forging or open die forging.

 

5. Non-ferrous Metals and Alloys (Non-Ferrous Metals & Alloys) - Special applications

- Aluminum alloys:

- Extremely lightweight (high weight reduction requirements).

- Low-speed and low-load gears in aviation, racing, and portable equipment. - It has lower strength and wear resistance than steel and is commonly used as small gears for auxiliary parts of clamps and brackets.

- Copper alloys (such as high-strength bronze):

- Excellent corrosion resistance (in seawater environments).

- Good wear resistance and low friction coefficient (anti-seizing).

- Used in specific instruments, underwater equipment, and situations requiring chemical corrosion resistance (as small gears or mating materials for large gears, such as worm wheels).

- For example, high-strength beryllium bronze (C17200, Alloy 25) can achieve strength close to steel after aging hardening.

- Stainless steel (such as precipitation-hardened stainless steel):

- Extremely high corrosion resistance requirements (chemical industry, medical devices, food industry).

- For example, 17-4PH (AISI 630, 06Cr17Ni4Cu4Nb) can obtain high strength (about HRC40-45) and good corrosion resistance through heat treatment.

- Used in instrument gears or small transmission parts operating in corrosive environments.

 

China Vigor has more than 20 years experience and the strong technical team and production capabilities in die-forging. If you have any question, demand, related parts development or improve your supply chain, please feel free to contact us info@castings-forging.com