Differences Between 316 and 316L Stainless Steel and Their Applications in CNC Machining
Content Menu
● 1. Welding method
● 2. Welding materials
● 3. Welding parameters
● 4. Groove form and assembly positioning welding
● 5. Welding technical requirements
● 6. Results
● What is the difference between 316 and 316L in CNC machining difficulty?
● Machinability:
● Tool Wear:
● Heat Generation:
● Weldability:
316L is a stainless steel material grade. The corresponding American grade is AISI 316L, while the Japanese equivalent is SUS 316L. In my country, the current standard grade is 022Cr17Ni12Mo2, and the previous standard was 00Cr17Ni14Mo2. These designations indicate that the steel primarily contains chromium (Cr), nickel (Ni), and molybdenum (Mo), with the numbers representing their approximate percentages.
So do you know the difference between 316 and 316L?
First, let’s take a look at the chemical composition of the material.
We can see that: the carbon content of the two is different, 316L has a lower carbon content and better corrosion resistance.
1. Welding method
Since most of the stainless steel pipes on site are of different sizes, we aim to minimize heat input based on the welding characteristics of stainless steel. For pipes with a diameter greater than 159 mm, we use argon arc welding for the base layer and manual arc welding for the cover. For pipes with a diameter of less than 159 mm, we use only argon arc welding. The welding machine utilized is an inverter arc welding machine WS7-400, which can be used for both manual arc welding and argon arc welding.
2. Welding materials
Austenitic stainless steel is a type of high-performance steel. To ensure optimal joint performance, the welding material should be chosen based on the “equal composition” principle. Additionally, to improve the joint’s resistance to thermal cracking and intergranular corrosion, a small amount of ferrite is included in the joint. For this purpose, H00Cr19Ni12Mo2 argon arc welding wire and the CHSO22 manual arc welding electrode are recommended as filling materials. Their compositions are detailed in Tables 1 and 2.
3. Welding parameters
Austenitic stainless steel has a key characteristic: it is sensitive to overheating. Therefore, welding should be done with low current and rapid techniques. When welding multiple layers, it’s crucial to keep the interlayer temperature below 60 °C. For specific parameters, please refer to Table 3.
4. Groove form and assembly positioning welding
The groove design features a V-shaped groove. By utilizing a smaller welding current and achieving less penetration, the blunt edge of the groove is narrower compared to carbon steel, measuring approximately 0 to 0.5 mm. Additionally, the groove angle is greater than that of carbon steel, ranging from about 65° to 70°. This design is illustrated in Figure 1.
Due to the high thermal expansion coefficient of stainless steel, significant welding stress is generated during the welding process, necessitating precise positioning for effective welding.
- For pipes with a diameter of up to 89 mm, two-point positioning should be used.
– For pipes with a diameter between 89 mm and 219 mm, three-point positioning is recommended.
– For pipes with a diameter of 219 mm or more, four-point positioning is required.
The length of the positioning weld should be between 6 to 8 mm.
5. Welding technical requirements
– For manual arc welding, the welding machine uses a DC reverse connection. In contrast, for argon arc welding, a DC positive connection is used.
- Before welding, brush off any surface oxide scale from the welding wire with a stainless steel wire brush and clean it with acetone. Additionally, the welding rod should be dried at a temperature of 200-250°C for 1 hour before use.
- Prior to welding, remove any oil and dirt from a 25 mm area on both sides of the groove of the die casting aluminum workpiece. This 25 mm area should also be cleaned with acetone.
- For argon arc welding, use a nozzle with a diameter of 2 mm and a cerium tungsten electrode with a diameter of 2.0 mm.
- When performing argon arc welding on stainless steel, it is essential to protect the backside with argon gas to ensure proper backside formation. Use the method of localized argon filling in the pipeline, maintaining a flow rate of 5-14 L/min. The front argon gas flow rate should be set between 12-13 L/min.
Note：
① During base welding, the weld thickness should be kept as thin as possible, and it should be well fused with the root. When shutting off the arc, ensure it forms a gentle slope shape. If any arc shrinkage holes are present, they should be ground off using a grinder. Always start and extinguish the arc within the groove. When the arc is extinguished, make sure to fill the arc pit to prevent the formation of arc pit cracks.
② Since stainless steel is an austenitic type, it is important to control both the interlayer temperature and the post-weld cooling rate to prevent carbide precipitation sensitization and intergranular corrosion. The interlayer temperature during welding should be kept below 60°C. Additionally, water cooling must be conducted immediately after welding, and a segmented welding approach should be utilized. The specific method for segmentation is illustrated in Figure 2. This symmetrically distributed welding sequence helps increase the cooling rate of the joint and reduces welding stress.
6. Results
– The appearance inspection showed no defects such as pores, weld nodules, depressions, and undercuts, and the forming was good.
- The specimens underwent tensile and bending tests, meeting all mechanical performance indicators without any defects such as lack of fusion or cracks.
- Macroscopic metallographic inspection revealed that the weld was well-fused, with a penetration depth of 1 to 1.5 mm. Microscopic metallographic analysis indicated that both the base material and the heat-affected zone exhibited an austenitic structure. The weld metal, however, consisted of a mixed structure of austenite and 4% ferrite, which fully met the requirements for intergranular corrosion resistance and resistance to embrittlement. The quality of the welding project was ensured through on-site construction at the coal chemical company.
What is the difference between 316 and 316L in CNC machining difficulty?
Machinability:
316L is generally easier to machine than 316 because it has a lower carbon content. This reduced carbon content decreases the likelihood of carbide precipitation, making 316L less prone to hardening, which is beneficial for machining in terms of tool wear and surface finish.
On the other hand, 316 has a higher carbon content, which can make it harder and more susceptible to work hardening during customized CNC machining. As a result, 316 can be more challenging to work with, especially in operations that generate heat, such as cutting or milling.
Tool Wear:
Both materials can cause significant tool wear due to their high strength and toughness, but 316 may cause slightly more wear because of its higher carbon content, especially in high-speed operations.
Heat Generation:
When machining 316 stainless steel, there is a slightly higher tendency for work hardening, which can generate additional heat. As a result, it may require more cooling and slower feed rates to prevent thermal damage.
In contrast, 316L stainless steel generates less heat and is less susceptible to work hardening. This makes it easier to manage tool temperatures and enhances overall machining efficiency.
Weldability:
316L is commonly preferred for welding applications because it has a lower carbon content. This characteristic reduces the risk of carbide precipitation and minimizes corrosion at the weld joints. Although this is not a difference related to machining itself, it can impact the overall design and application considerations for CNC machining components being machined from either material.
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