Are you not sure which steel to use for your project? After all, in CNC machining services, the use of steel is related to the quality and performance of the finished product. And as popular alloy steels, 4140 steel and 8620 steel have their own properties and uses. However, I can tell you that 4140 is suitable for overall quenched and tempered parts that require high strength and toughness, and 8620 is suitable for carburized parts that require an extremely hard surface but a tough core. If you want to know exactly how the two differ, I’ll explain below.
What is 4140 steel?
4140 steel can be called the workhorse of the industry. It is a medium carbon chromium-molybdenum alloy steel. It is named as such because ’41’ indicates the alloy series in steel contains chromium and molybdenum elements. At the same time, ’40’ also signifies that the carbon content of this steel is approximately 0.40%. In addition, this steel is also known as modulated steel. It is often used in a quenched and high-temperature tempered state. It is precisely because of the addition of chromium and molybdenum that AISI 4140 has excellent tensile strength and fatigue resistance. Meanwhile, in terms of comprehensive mechanical properties, its performance is also quite good.
What is 8620 Steel?
8620 steel is a low-carbon nickel-chromium-molybdenum alloy steel. It is the most classical representative of penetration steel. Among them, the meaning of ‘86’ refers to its alloy series, including nickel (Ni), chromium (Cr) and molybdenum (Mo). The ‘20’ represents its carbon content, which is about 0.20%. It is precisely because of the low carbon content of this steel that 8620 steel cannot be hardened very hard and must be used through the carburizing process. So in terms of characteristics, the surface of 8620 steel has extremely high hardness and wear resistance, but its core also has excellent toughness. In addition, 8620 steel also has excellent welding properties.
4140 vs. 8620 Chemical Composition Comparison
| Element | AISI 4140 (wt%) | AISI 8620 (wt%) | Engineer’s Note |
| Carbon (C) | 0.38 – 0.43 | 0.18 – 0.23 | The key difference. 4140’s higher carbon content provides high overall strength. 8620’s lower carbon content provides better weldability and excellent core toughness. |
| Nickel (Ni) | – | 0.40 – 0.70 | Nickel greatly improves toughness and impact resistance, especially at low temperatures. |
| Chromium (Cr) | 0.80 – 1.10 | 0.40 – 0.60 | Found in both. Chromium mainly adds to hardenability (depth of hardness) and wear resistance. |
| Molybdenum (Mo) | 0.15 – 0.25 | 0.15 – 0.25 | Found in both. Molybdenum assists in maintaining strength at high temperatures and also prevents temper brittleness. |
4140 vs. 8620 Mechanical Properties
1. Hardness
The distribution of hardness on 4140 is relatively uniform. It is generally in the form of pre-hardened steel (HRC 28-32) or heat-treated to HRC 50-55. 8620 is extremely wear-resistant on the surface and can reach up to HRC 60-64 in hardness, but its core hardness is only around HRC 30-40.
2. Strength
4140 has greater tensile strength and yield strength. If your parts are required to resist tensile, bending or torsional loads, then 4140 is a better choice, because it is a stronger material overall. 8620 only has an extremely high contact fatigue strength on its surface.
3. Toughness
4140 has good toughness but can become brittle at very high hardness. The 8620 core has excellent toughness. Therefore, even if cracks appear on its surface, the soft core can prevent the cracks from spreading rapidly and prevent the phenomenon of instantaneous fracture of parts.
4140 vs. 8620 Processability and Weldability
1. Machining
In terms of machining, the 8620 is very easy to machine in the annealed state and has better cutting performance than the 4140. The finish performance of 4140 is even better.
2. Welding
8620 has good welding properties due to the low carbon content and generally does not need to be complex preheated. Due to its high carbon content and poor weldability, 4140 is prone to cracks during welding and must be strictly preheated and post-weld heat treated.
4140 vs. 8620 Application
Scenario using 4140 steel
Your parts are primarily used in environments subjected to torsional, bending, or tensile forces. If these parts are too soft internally, plastic deformation or distortion may occur under high torque.
Examples of parts: crankshafts, connecting rods, heavy-duty shafts, hydraulic cylinder piston rods, high-strength bolts, milling cutter rods.
Scenario using 8620 steel
If the surface of your parts needs extremely high wear resistance and also needs to endure very large impact loads, then you can choose 8620 steel. This type of steel has a structure that is hard on the outside and tough on the inside.
Examples of parts: Pinion gears, camshafts, piston pins, chain pins.
4140 vs. 8620 Heat Treatment
AISI 4140: Overall Strengthening
Process: Heating → quenching (usually oil quenching) → High temperature tempering
This is a relatively straightforward process and does not require lengthy chemical penetration. The composition of 4140 alloy has good hardenability, and after heat treatment of the material, the difference in hardness from the surface to the center is small.
Pre-hardened: 4140 is often sold at HRC 28-32. This is a huge advantage and means that you can process the finished product directly, completely eliminating the subsequent heat treatment steps.
AISI 8620: Surface Modification
Process: Carburizing (heating in a carbon-rich atmosphere for a long time to allow carbon atoms to penetrate) → Quenching → Low temperature tempering.
Heat treatment of 8620 is very time-consuming as the carbon atoms diffuse slowly into the surface of the steel. In order to achieve a deeper carburized layer, the part needs to remain in the furnace for several hours or even longer. And you also need to precisely control the atmosphere, temperature and time to ensure the thickness and carbon concentration of the carburized layer.
Heat Treatment Comparison Table
| Factor | AISI 4140 (Direct Hardening) | AISI 8620 (Case Hardening) |
| Heat Treat Process | Quench & Temper (Q&T) | Carburizing |
| Process Cycle Time | Short (Hours) | Long (Can be 10+ Hours) |
| Machinability | Moderate (if Pre-hardened) | Excellent (in Annealed state) |
4140 vs. 8620 Steel Cost
1. Raw Material Cost
From the perspective of raw materials, the price of 8620 is higher. This is because 8620 contains nickel, an expensive alloy element. In relative terms, 4140 is mainly composed of chromium and molybdenum, which is relatively cheap.
2. Processing Cost
In terms of processing cost, 8620 is more expensive than 4140. This is because the carburizing process of 8620 consumes a lot of energy and time, which also increases the furnace time cost. And if you purchase pre-hardened materials, the heat treatment cost of 4140 is 0.
Parts Distortion Control: 4140 vs. 8620 Steel
Do you have such a problem? The tolerances of the parts are designed, but after these parts are heat treated, the tolerances deviate greatly, resulting in a high scrap rate of the parts. This kind of problem is actually relatively common. This is because you have not considered the thermal processing risk of 8620 steel.After all, during the entire heat treatment process, 8620 needs to be maintained at a high temperature (900°C+) for a long time and then quenched. This long-term high temperature stay coupled with phase change can easily lead to warpage or elliptical deformation of precision parts. Therefore, when you choose 8620, it often means that you must reserve a grinding allowance.
At this time, the advantages of 4140 are reflected, because you can directly purchase pre-hardened 4140 steel. In this case, you can directly process the finished product to size without the need for subsequent heat treatment. This completely avoids the risk of deformation.
Why 8620 Outperforms 4140 in Fatigue Life?
After 4140 is quenched and tempered, the surface usually does not have beneficial compressive stress, so if there are microscratches on its surface, the cracks can easily spread. However, 8620 performs differently. This is because carburizing is not just about increasing hardness, it also has a hidden property – surface residual compressive stress. This pressure can effectively close microcracks and greatly improve the high cycle fatigue life. This is the reason why most gears are made of 8620.
Summary
I believe that by reading the entire article, you already have the answer to which steel material to choose for your project. But our suggestion is, don’t blindly choose 8620 for the pursuit of high-performance parts. If the hardness of 4140 is enough to cope with wear and other needs, then you should choose 4140. And you will also find that you save 20% – 30% of the overall manufacturing cost and the delivery time is shorter. Of course, if you are still hesitant about material selection, you can also contact us. Our DSR company is a professional CNC machining manufacturer, and our team of engineers will provide you with a satisfactory solution or custom steel machining service!
FAQ
Q1: Can 8620 and 4140 steel be used interchangeably?
A: They can not be used interchangeably easily in most cases. It mainly depends on the parts to be made, whether the resistance is the main requirement, or whether the surface wear resistance and internal impact resistance are the main requirements. Choose 4140 for the former and 8620 for the latter.
Q2: Is 4140 the same as B7?
A: 4140 and B7 are not the same thing, but the chemical composition of B7 bolt material is roughly equivalent to that of Cr-Mo alloy steel such as 4140/4142.
Q3: What steel is stronger than 4140?
A: In the same system as 4140, there are Ni-Cr-Mo ultra-high-strength steels such as 4340 and 300M (4340M), which are stronger. Some other steels include typical carburized gear steels such as 20CrNiMo and 18CrNiMo7‑6.
Q4: Is 4140 the same as EN19?
A: 4140 and EN19 are basically the same type of steel: they are both medium carbon Cr‑Mo alloy steels with very similar chemical compositions and properties.
Q5: Is A36 the same as 4140?
A: A36 and 4140 are actually completely different types of steel. A36 is a low-carbon ordinary structural steel with low strength, while 4140 is a medium carbon Cr-Mo alloy steel that can be quenched and tempered to high strength.
Q6: What is the maximum hardness of 8620 steel?
A: The surface hardness of 8620 steel after normal carburizing and quenching is generally HRC 58–62. After aggressive carburizing and quenching, the local level can be close to HRC 63.
