8620 steel’s properties make it stand out as one of the most versatile alloys used in engineering today. This carburizing alloy steel shows remarkable strength with tensile measurements of 530 MPa (76,900 psi) and yield strength reaching 385 MPa (55,800 psi). The steel’s Brinell hardness measures 149 in its annealed state and delivers outstanding results in tough industrial settings.
AISI 8620 shines when you need high corrosion and wear resistance along with solid core strength. Engineers often pick this alloy for equipment that needs to last, especially when you have oil drilling operations where durability is vital. The steel’s unique makeup gives it a great machinability rating of 65, compared to the baseline 100 for AISI 1212 steel. You’ll find its fatigue strength impressive too, ranging from 270 to 360 MPa. The material works really well with heat treatment processes, which makes it perfect for different manufacturing methods like metal injection molding. This detailed piece explains why 8620 steel remains the top choice for engineers who just need reliable performance in vital applications.
Detailed 8620 Steel Composition and Microstructure
AISI 8620 steel stands out because of its balanced chemical makeup that delivers excellent performance in tough applications. This low-carbon, triple-alloyed steel contains specific amounts of alloying elements that boost its microstructural characteristics and mechanical properties significantly.
Nickel, Chromium, and Molybdenum Content Breakdown
ASTM A29 sets strict standards for AISI 8620 steel’s chemical composition. The steel contains 0.18-0.23% carbon, 0.7-0.9% manganese, 0.4-0.7% nickel, 0.4-0.6% chromium, and 0.15-0.25% molybdenum. These elements maintain balanced proportions with 0.15-0.35% silicon. Sulfur and phosphorus levels stay below 0.040% and 0.035% respectively. Iron makes up the rest, about 96.9-98.0% of the alloy.
The nickel content boosts core toughness, while chromium adds to hardenability and wear resistance. Molybdenum, though in smaller amounts, plays a vital role in preventing temper brittleness and improving high-temperature strength. These elements work together to create an alloy with exceptional case hardening capabilities.
Ferrite and Pearlite Phases in the Annealed State
AISI 8620 steel’s annealed condition shows a unique microstructure with ferrite and pearlite grains. The normalized heat treatment process creates this microstructure and typically produces an average hardness of 83 HRB. The way these phases are arranged affects the material’s mechanical properties directly.
The steel shows impressive versatility despite its simple annealed state microstructure. Ferrite provides ductility and toughness, while pearlite adds strength. The annealing process heats the steel to 820-850°C until it reaches uniform temperature throughout, followed by furnace or air cooling.
Effect of Carburizing on Microstructure
Carburizing revolutionizes 8620 steel’s microstructure. The surface layer soaks up carbon at high temperatures and creates a carbon gradient from surface to core. The microstructure changes from ferrite-pearlite to a complex phase arrangement.
The high-carbon surface layer turns into hard martensite with retained austenite after carburization and quenching. The core stays tougher with a lower-carbon structure. This change raises surface hardness and wear resistance by a lot while keeping the core tough. Studies show that carburization can boost ultimate tensile strength by up to 138.1%.
Heat treatment creates a gradient of microstructural changes. Molten regions completely convert to martensite. The heat-affected zone shows pearlite transformation but leaves ferrite unchanged outside the complete martensite transformation zone.
Mechanical and Thermal Properties of AISI 8620
AISI 8620 steel’s mechanical and thermal properties change based on heat treatment conditions. This makes it an excellent choice for engineering applications that need specific strength-to-weight ratios.
Tensile Strength: 530 MPa in Annealed Condition
The steel shows a tensile strength of 530 MPa (76,900 psi) and a yield strength of 385 MPa (55,800 psi) in its annealed state. Heat treatment increases these values significantly—the ultimate tensile strengths reach 1050 MPa (152,000 psi) to 1157 MPa (168,000 psi) based on the treatment protocol. The elongation at break varies from 14.3% to 31%, showing the material’s excellent balance of strength and ductility. The tensile strength of carburized and oil-quenched components ranges from 690 MPa to 1270 MPa, depending on section size.
Hardness Range: 149HB to 155HRC Post-Treatment
Heat treatment creates dramatic changes in 8620 steel’s hardness values. The annealed steel has a Brinell hardness of 149, which equals about 80 on the Rockwell B scale and 155 on the Vickers scale. After case hardening, the surface hardness can reach 61 HRC, while the core stays at 35-45 HRC. This creates an ideal mix of wear resistance and impact toughness from surface to core.
Thermal Conductivity: 46.6 W/mK
8620 steel’s thermal conductivity measures 46.6 W/mK (323 BTU-in/hr-ft²-°F), typical for low-alloy steels. This value stays stable across different heat treatments, which ensures reliable performance when temperatures fluctuate.
Elastic and Shear Modulus Values
The steel’s elastic modulus ranges between 190-210 GPa, with 205 GPa (29,700 ksi) as the standard value. The material has a shear modulus of 80 GPa (11,600 ksi) and a bulk modulus of 140 GPa (20,300 ksi). Its Poisson’s ratio falls between 0.27 and 0.30, though 0.29 is the most widely accepted value. These properties determine how the material behaves under load and its usefulness in structural applications.
Heat Treatment and Case Hardening Behavior
Heat treatment changes how 8620 steel performs and makes it suitable for industrial applications that need specific mechanical properties.
8620 Heat Treatment Cycle: Annealing vs Normalizing
The annealing process for 8620 steel needs heating to 820-850°C. The temperature must equalize throughout before slow cooling in a furnace. This brings down the hardness to about 149 HB and makes the steel easier to machine. The normalizing process needs higher temperatures of 900-925°C with air cooling. This results in stronger steel with tensile strength of ≥635 MPa compared to annealed steel’s ≥535 MPa.
Carburizing Temperature and Quenching Parameters
The carburizing process happens at 900-950°C in a carbon-rich environment. Carbon moves into the steel’s surface layer during this process. The steel then goes through core refining at 840-870°C and quenching in water, oil, or air. Case hardening needs another heat cycle at 780-820°C after core refining and oil quenching.
Core vs Surface Hardness After Case Hardening
After carburizing, 8620 steel’s surface hardness reaches 55-60 HRC. The core hardness stays between 35-45 HRC. This difference in hardness creates a perfect mix – a wear-resistant surface with a tough, flexible core.
Impact of Tempering on Toughness and Wear Resistance
Tempering between 150-200°C makes both case and core tougher without losing much surface hardness. This makes the steel less likely to crack during grinding and improves its resistance to wear.
Metal Injection Molding and Machinability Considerations
8620 steel’s manufacturing versatility makes it stand out for advanced production methods. This alloy goes beyond traditional manufacturing and is a chance to excel in specialized applications.
Suitability of 8620 for Metal Injection Molding
8620 steel shines in metal injection molding processes because of its excellent formability and balanced mechanical properties. The process mixes fine metal powder with waxes and polymers to create complex parts with precise dimensions. MIM-8620 components reach densities of ≥7.45 g/cm³ and tensile strengths of ≥450 MPa after sintering. We used it mainly in automotive components, weapons systems, and precision machinery. The material costs nowhere near as much as stainless steel powders like 316L, with savings of 15-25%.
Machinability Index: 65% Compared to AISI 1212
AISI 8620 steel shows a machinability rating of 66% compared to AISI 1212 steel’s baseline of 100%. These numbers mean recommended cutting speeds should be around 30 m/min versus 50 m/min for AISI 1212. The material machines better in its annealed state.
Post-Heat Treatment Machining Constraints
Machining options become limited after heat treatment or carburizing. You should keep operations minimal to protect the hardened case. Grinding remains the best option for post-carburized components.
Welding and Grinding Limitations
8620 steel works well with MIG and TIG welding using ER70S-6 and ER80S-Ni filler metals. Preheating at 150-200°C helps prevent cracking in thicker sections. The material needs post-weld heat treatment to relieve stress and restore toughness.
Conclusion
8620 steel has proven to be a versatile engineering alloy with properties that make it essential for critical applications. The balanced mix of nickel, chromium, and molybdenum creates a material that performs well in tough environments. These elements work together to give the alloy impressive strength values – 530 MPa tensile strength and 385 MPa yield strength in its annealed state.
Heat treatment substantially changes 8620 steel and lets engineers customize its properties for specific uses. The surface hardness reaches 55-60 HRC after carburizing, while the core stays tough at 35-45 HRC. This creates a perfect blend of wear resistance and impact toughness. On top of that, it has thermal conductivity of 46.6 W/mK that gives reliable performance in different temperature conditions.
The ability to manufacture 8620 steel easily gives it another advantage. The alloy works well in metal injection molding processes despite having a lower machinability index of 65% compared to AISI 1212 steel. MIM-8620 parts achieve excellent densities and mechanical properties. They cost 15-25% less than stainless steel options, which makes them cost-effective for automotive, weapons systems, and precision machinery.
The steel’s case hardening abilities, core strength and wear resistance make it a top choice for engineers who design critical components. This alloy delivers consistent results under extreme conditions – from oil drilling equipment to automotive gears. Engineers will without doubt keep choosing 8620 steel when they need the right mix of surface hardness, core toughness, and manufacturing flexibility beyond 2025.
Key Takeaways
Engineers choose 8620 steel for its exceptional balance of strength, versatility, and cost-effectiveness in critical applications.
• Superior mechanical properties: 8620 steel delivers 530 MPa tensile strength in annealed state, reaching up to 1157 MPa after heat treatment with excellent fatigue resistance.
• Exceptional case hardening capability: Carburizing creates surface hardness of 55-60 HRC while maintaining tough 35-45 HRC core, ideal for wear-resistant applications.
• Manufacturing versatility: Offers 65% machinability rating and excels in metal injection molding, costing 15-25% less than stainless steel alternatives.
• Balanced alloy composition: Nickel, chromium, and molybdenum content provides optimal corrosion resistance, hardenability, and prevents temper brittleness.
• Heat treatment flexibility: Responds exceptionally well to various treatments, allowing engineers to tailor properties for specific applications from automotive to oil drilling equipment.
This combination of properties makes 8620 steel the go-to choice for engineers requiring reliable performance in demanding environments where both surface durability and core toughness are essential.
FAQs
Q1. What are the primary applications of 8620 steel? 8620 steel is widely used in manufacturing medium-strength parts such as forged camshafts, gears, and fasteners. Its exceptional case hardening capabilities and substantial core strength make it ideal for components in oil drilling equipment, automotive parts, and other applications requiring high wear resistance and toughness.
Q2. What are the key mechanical properties of 8620 steel? In its annealed state, 8620 steel has a tensile strength of 530 MPa and a yield strength of 385 MPa. After heat treatment, tensile strength can reach up to 1157 MPa. It also has a density of 7.85 g/cm³ and demonstrates excellent fatigue resistance, making it suitable for demanding industrial environments.
Q3. How does heat treatment affect 8620 steel’s properties? Heat treatment significantly enhances 8620 steel’s performance. Carburizing can increase surface hardness to 55-60 HRC while maintaining a tough core of 35-45 HRC. This process creates an ideal combination of wear resistance on the surface and impact toughness in the core, allowing for tailored properties in specific applications.
Q4. What makes 8620 steel suitable for metal injection molding (MIM)? 8620 steel excels in metal injection molding due to its excellent formability and balanced mechanical properties. MIM-8620 components can achieve densities of ≥7.45 g/cm³ with tensile strengths reaching ≥450 MPa. Additionally, it’s cost-effective, being 15-25% less expensive than stainless steel powders like 316L for MIM processes.
Q5. How does 8620 steel’s machinability compare to other steels? 8620 steel has a machinability rating of 65% compared to the baseline 100% for AISI 1212 steel. While this indicates slightly more challenging machining, 8620 steel still offers good machinability, especially in its annealed state. Its versatility in manufacturing processes, including both traditional machining and advanced techniques like MIM, makes it a preferred choice for many engineering applications.