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A Comprehensive Guide to 310 MoLN Steel

What is 310 MoLN steel?

310 MoLN (UNS S31050/1.4466/725LN) is a urea grade ultra-low carbon austenitic stainless steel with a density of 7.9 g/cm3. Urea-grade stainless steel is used in urea production equipment at high temperature and high pressure in strong corrosive medium of ammonium methylate solution, so the requirements are very strict.

20230804135640 31463 - A Comprehensive Guide to 310 MoLN Steel

Characteristics of 310 MoLN steel

  • Good corrosion resistance, including corrosion resistance in high-temperature and high chloride environments;
  • Good antioxidant and heat resistance properties, which can be used for a long time at high temperatures;
  • Good plasticity and processability, easy to machine into various shapes of parts and components.

Like other commonly used austenitic stainless steels, such as 304 or 316, 725LN has good cold and Hot working properties. The difference is that the strength of 725LN is slightly higher than that of 304 or 316, requiring greater deformation force.

Chemical Composition of 310 MoLN steel

Standard Steel Grade
Chemical Composition %
C: Mn: Si: P: S: Cr: Mo: Ni: N:
EN 1.4466 – X1CrNiMoN25-22-2
<0.02 <2.0 <0.7 <0.025 <0.010 24.0 – 26.0 2.0 – 2.5 21.0 – 23.0 0.10 – 0.16
ASTM UNS S31050 – AISI 310MoLN – 25.22.2
<0.025 <2.0 <0.5 <0.020 <0.030 24.0 – 26.0 1.6 – 3.0 20.5 – 23.5 0.09 – 0.16
NF Z2CND25-22Az
<0.02 <2.0 <0.75 <0.025 <0.010 24.0 – 26.0 2.0 – 2.5 21.0 – 23.0 0.10 – 0.16
BS 725LN
<0.02 <2.0 <0.7 <0.025 <0.010 24.0 – 26.0 2.0 – 2.5 21.0 – 23.0 0.10 – 0.16
SEW 400 1.4465 – X 1 CrNiMoN 25-25-2 – X1CrNiMoN25-25-2
<0.02 <2.0 <0.7 <0.020 <0.015 24.0 – 26.0 2.0 – 2.5 22.0 – 25.0 0.08 – 0.16
GOST 02Ch25N22AM2 – 02Х25Н22АМ2
<0.02 1.5 – 2.0 <0.4 <0.020 <0.015 24.0 – 26.0 2.0 – 2.5 21.0 – 23.0 0.10 – 0.14

Mechanical Properties of 310 MoLN steel

  • Tensile strength σb (MPa): 540 – 570 MPa
  • Yield strength σs (MPa): >250 MPa
  • Elongation δ5 (%): > 40%
  • Impact energy Akv (J): > 60
  • Linear expansion coefficient, α: 15.7 * 10-6 K-1
  • Modulus of elasticity, E: 190 GPa
  • Thermal capacity, cp: 500 J * kg-1 * K-1
  • Thermal conductivity, λ: 15 W * m-1 * K-1
  • Hardness: ≤ 240HB

Physical Properties of 310 MoLN steel

Density Specific heat capacity Thermal conductivity Electrical resistivity
g/cm3 J/kg.K W/m.K mm2/m
7.9 500 14 0.8

Heat treatment specification

  • Supersaturation at 1070-1150 ℃.
  • Rolling and forging at a temperature of 1150-850℃.
  • Heat treatment ( 1140-1180°C/2084-2156°F ) and water quenching.

General corrosion of 310 MoLN (UNS S31050)

  • 310 MoLN (UNS S31050) was originally developed for stripper tubes used in the production of urea.
  • 310 MoLN (UNS S31050) has excellent corrosion resistance in urea/carbamate solutions at high pressures and temperatures.
  • 310 MoLN (UNS S31050) is also highly resistant to inorganic acids.

725LN is a high alloyed urea grade austenitic stainless steel with a very low carbon content. The steel is developed to solve the selective corrosion of hot Ammonium carbamate (AMAC) in urea production.
As a urea grade stainless steel, emphasis is placed on intergranular corrosion and selective corrosion in terms of corrosion.
Reducing the carbon content in steel is the key to preventing intergranular corrosion. When it decreases to below 400-850 ℃ (the solubility limit of carbon in austenite), preventing the precipitation of Cr carbides can effectively prevent intergranular corrosion. When m (C) ≤ 0.03%, intergranular corrosion will not occur in the base metal or weld seam in a corrosive medium between 400 and 850 ℃.
Selective erosion is related to corrosive media and metallographic structure, in media such as sulfuric acid or urea δ- The ferrite phase will preferentially corrode. For such media, pure austenite structure has the best corrosion resistance. Due to the extremely low ferrite content of 725LN, it has a nearly complete austenite structure. Therefore, this type of steel will not undergo selective erosion. Urea grade stainless steel should pass the ASTM A-262 “E” Huey intergranular corrosion test and selective corrosion test.

Intergranular corrosion of 310 MoLN (UNS S31050)

310 MoLN (UNS S31050) is highly resistant to intergranular corrosion after welding.

Pitting and crevice corrosion of 310 MoLN (UNS S31050)

310 MoLN (UNS S31050) has very good resistance to pitting, and is also far more resistant to crevice corrosion than ASTM 316L.

Stress corrosion cracking (SCC) of 310 MoLN (UNS S31050)

Conventional austenitic stainless steels of type ASTM 304 and 316 are susceptible to stress corrosion cracking (SCC) in chloride- bearing solutions at temperatures exceeding about 60°C (140°F). The higher nickel content makes 310 MoLN (UNS S31050) slightly more resistant.

Erosion corrosion of 310 MoLN (UNS S31050)

The good mechanical strength of 310 MoLN (UNS S31050) makes it resistant to erosion-corrosion. Ferrules for urea strippers are one application where this property is utilised.

Weldability of 310 MoLN (UNS S31050)

  • The weldability of 310 MoLN (UNS S31050) is good. Welding must be carried out without preheating, and normally there is no need for any subsequent heat treatment.
  • 310 MoLN (UNS S31050) has low thermal conductivity and high thermal expansion.

Selection of welding methods

Due to the high element content of the 725LN alloy, tungsten argon arc welding should be used as much as possible on the side in contact with the medium. Due to the high temperature and concentrated heat of argon arc, as well as the cooling effect of argon gas flow, the cooling speed of the weld seam is faster, the heat affected zone is smaller, and the burning loss of alloy elements is also less, which is conducive to the final formation of a complete austenitic structure in the deposited metal of the weld seam. In addition, tungsten argon arc welding does not generate splashes and has a good surface, which is beneficial for corrosion prevention.
However, for this equipment, the plate thickness is 8mm, and it is uneconomical to use only argon tungsten arc welding. The combination method of argon tungsten arc welding backing, welding rod Arc welding filling and covering should be used, which not only avoids weld gouging, but also improves welding efficiency.

Selection of welding materials

Filler metal
Sandvik25.22.2.LMn (AWSA5.9ER “310LMo”) is used as the welding wire, which is the product of Sandvik Company in Sweden.
The welding rod is Avesta 254 SFER (equivalent to the BM310MoL of Filarc), which is a welding material matching 725LN produced by Avista in Sweden.
These two welding materials can also be used for the welding of urea grade stainless steel 316Lmod or 304, 316, or stainless steel that requires no magnetism after welding.

To prevent overheating of welded joints, small diameter welding materials should be used, which can reduce the welding current, thereby reducing line energy and reducing heat input to the welded joints. For 8mm thick plates, optional Φ 2.4 welding wire and Φ 4.0 welding rods.
The welding wire must be clean and wiped with acetone before use.
Avesta 254 SFER welding rod needs to be dried at 250 ℃/3h.
Protective gas
Industrial pure argon is commonly used as the protective gas, and 30% He or 1-5% H2 can be added to increase arc energy, increase weld penetration and fluidity of the weld pool, and improve welding speed. Automatic welding often uses this mixed gas.

Uses of 310 MoLN Steel

310MoLN alloy is mainly used in the following fields:

  • Chemical and petrochemical industries, used to manufacture equipment and components that are resistant to corrosion, high temperature, and wear, such as reactors, heat exchangers, steam generators, pipelines, etc;
  • Flue gas desulfurization and denitrification system, used for manufacturing equipment and components that are resistant to corrosion, high temperature, and wear, such as injectors, oxidizers, catalysts, etc;
  • The pulp and paper industry is used to manufacture equipment and components that are resistant to corrosion, high temperature, and wear, such as steam generators, steam dryers, pulp tanks, etc;
  • The food and pharmaceutical industry is used to manufacture corrosion-resistant, high-temperature resistant, and pollution-free equipment and components, such as containers, pipelines, storage tanks, etc.

In summary, 310MoLN alloy has excellent corrosion resistance, high temperature resistance, and wear resistance, and can be used in various harsh industrial environments, providing reliable material support for industrial production.

Stainless steel 310 MoLN electroslag ingot forging cracks cause analysis and process improvement

A large number of surface transverse cracks were found on 20t-EAF-VOD-LF-VD-cast bar-2.5t electroslag remelting ingot forged into Φ200mm round bar, and a scanning electron microscope was used to observe the micro-morphology of the cracks, and the analysis results showed that the cracks were caused by the high heating temperature before forging, which led to the melting of grain boundaries. Through the test and production practice, 310 MoLN steel electroslag ingot heating temperature from the original 1180 ℃ down to 1110 ℃, eliminating the forging cracks.

310 MoLN belongs to ultra-low carbon nitrogen-containing austenitic stainless steel, with an alloy content of up to 50%, because of its good corrosion resistance, usually used in high pressure, high temperature, and strong corrosion environment.
This paper is mainly on the 310 MoLN stainless steel in the development process of forging cracks occurring in the cause of analysis, and through the temperature gradient experiment, found that the forging of the optimal heating temperature of the steel. By adjusting the forging heating temperature of 310 MoLN stainless steel, cracks are eliminated, and the final bar product reaches the national standard by inspecting various properties.

1. Material production process

310 MoLN stainless steel using 20t electric arc furnace + VOD + LF-VD smelting casting into 2.5t cast bar (section Φ410mm), 5t gas shielded electroslag remelting 2.5t ingot (section Φ500mm), 20MN fast forging billet + 16MN forging machine forging into Φ200mm round bar. The main components are shown in Table 1.
310 MoLN stainless steel in 20MN fast forging production heating temperature according to 1180 ℃ control, open forging temperature 1100 ℃, final forging temperature 930 ℃. Ingot started forging deformation control in a single side of 30mm or so; the surface began to appear tearing transverse cracks, see Figure 1 (a). Adjustment of forging under pressure to reduce to about 15mm; cracks continue to appear. The billet after air cooling was found in addition to the obvious large tear-like cracks, but also found in many holes. After removing the oxide skin with a grinding wheel, the holes’ surface was covered with fine cracks; cracks were distributed in a network, see Figure 1 (b). According to the analysis of forging production, the amount of underpressure is controlled within 20mm, the deformation is very small, and the forging process responds to the ingot’s lack of plasticity.

2. Crack detection and analysis

Cracks on the forging billet sampling and analysis, through the scanning electron microscope observation, in the austenite grain boundaries appeared in several cracks and holes, which is more typical of the overheating morphology. This indicates that the steel is heated at 1180 ℃, resulting in the formation of the superheated organization due to the high temperature, see Figure 2 (a). Through the holes in the incomplete melting of the grain boundary material composition and matrix composition comparison found that the melted material composition of ferrite-forming elements Cr, Mo content is high, austenite-forming elements Mn, Ni content is low, see Table 2, according to the Scheffler’s organization chart Cr/Ni equivalent calculation assessment and the metallographic results of the observation, the incomplete melting of the grain boundary material for the ferrite organization. Forging cracking is caused by the melting of the ferrite tissue at the austenite grain boundaries due to the high heating temperature, resulting in the formation of superheated tissue, see Figure 2(b).
Table.1 Chemical composition of 310 MoLN steel/%

Project C Mn Si P S Ni Cr Mo N
Standard composition ≤0.03 ≤2 ≤0.4 ≤0.03 ≤0.015 21.00-23 24.00-26 2.00-3 0.120-0. 160
Actual composition 0.01 1.16 0.1 0.01 0.002 21.59 24.36 2.21 0. 142

20230804142114 25825 - A Comprehensive Guide to 310 MoLN Steel

Figure.1 310 MoLN steel forging transverse cracks (a), the surface of the transverse fine cracks (b) morphology

20230804142233 85055 - A Comprehensive Guide to 310 MoLN Steel
Figure.2 310 MoLN steel forging crack morphology: (a) × 500; (b) × 5000, SEM
Table.2 Comparison of molten material and matrix composition/%

Region Cr Mn Fe Ni Mo
Molten substance 40.36 41.5 9.74 8.41
Matrix 27.11 1.57 45.74 21.82 3.77

Of which.

20230804142620 97048 - A Comprehensive Guide to 310 MoLN Steel

3. Heating experiment

For the 310 MoLN stainless steel forging heating phenomenon of overheating, in the 310 MoLN electroslag ingot end of the same part of the cut 4 pieces of 20mm × 20mm specimens in the same heating furnace were held at different temperatures for 3h after water cooling, grinding metallurgical specimens directly in the scanning electron microscope to observe the morphology of the grain boundaries, through the observation found that when the heating temperature of more than 1120 ℃, the austenite of the 310 MoLN steel Grain boundaries began to occur to varying degrees of melting, grain boundaries appear on the melting of the holes, was discontinuous (Figure 3). The higher the temperature, the more serious the melting of grain boundaries and the more holes in the grain boundaries.310 MoLN steel grain boundary corrosion test results are shown in Table 3.

4. Process improvement and effect

Through the forging cracking cause analysis and different heating temperature gradient experiments, the forging heating temperature from 1180 ℃ down to 1100 ℃ adjustment.

Table.3 Metallographic examination results of 310 MoLN steel heated at 1200℃, 1150℃, 1120℃ and 1100℃.

Number Sampling location Heating system Metallographic results
1 Tail of electroslag steel ingot, 20-40mm below the skin Heating temperature 1200 , insulation for 3 hours, water cooling. Continuous pores at grain boundaries
2 Tail of electroslag steel ingot, 20-40mm below the skin Heating temperature 1150 , insulation for 3 hours, water cooling ° Intermittent pores at grain boundaries
3 Tail of electroslag steel ingot, 20-40mm below the skin Heating temperature 1120 , insulation for 3 hours, water cooling. Intermittent pores at grain boundaries
4 Tail of electroslag steel ingot, 20-40mm below the skin Heating temperature 1100 , insulation for 3 hours, water cooling. No pores at grain boundaries

20230804143440 27849 - A Comprehensive Guide to 310 MoLN Steel
Fig.3 Grain boundary morphology of 310 MoLN steel heated at (a) 1200°C, (b) 1150°C, (c) 1120°C and (d) 1080°C

Table.4 Comparison of forging process parameters before and after adjustment

Forging process Heating temperature/°C Insulation time/h Forging equipment Reduction control/mm Surface states
Before adjustment 1180 3 20MN fast forging machine 15-30 Surface transverse cracks
After adjustment 1100 3 20MN fast forging machine 40-80 No surface cracks

Table.5 Test Properties of 310 MoLN Steel Forgings

Project Rp0.2/MPa Rm/MPa A/% Z/% Intergranular corrosion Hugh’s experiment
Standard ≥270 580-780 ≥25 ≥40
Detection value 302/302 638/636 46.5/48.0 76/78 Qualified Qualified

The same mother furnace electroslag ingot was selected to be re-produced in 20MN fast forging machine; the bilateral underpressure was controlled at 40-80mm during ingot forging deformation, no forging cracks were found on the surface of the material, and no surface cracks were found in the inspection of the steel after the metal material was formed (Table 4).

5. Inspection results

After adjusting the heating temperature of the forging process, the material was successfully forged. Through the inspection of the metal material, all the properties meet the requirements of the standard. At the same time on the high magnification of the organization observation, no ferrite and no cracks were found at the grain boundaries (Table 5).

6. Conclusion

  • (1) 310 MoLN stainless steel forging cracking is due to the heating temperature being too high at the grain boundaries of ferrite melting, leading to forging cracking. The higher the heating temperature, the more serious the melting of ferrite at grain boundaries.
  • (2) Through the heating temperature gradient experiment, 310 MoLN steel at a temperature of more than 1120 ℃, the grain boundaries of ferrite overheating and overcooking and local melting, the heating temperature control at 1100 ℃ below.
  • (3) 310 MoLN steel forging heating temperature control at 1100 ℃, the forging process did not find a cracking phenomenon after the material after the performance indicators to meet the standard requirements.

Author: Zhang Jun

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