A Comprehensive Guide: What is Urea Class Stainless Steel?

1. How is urea produced?

Urea production is synthesized from CO₂ and ammonia under high pressure (14-25 MPa) and 180-210 °C while generating ammonium methyl and water. The urea solution is purified, concentrated, and granulated to form granular urea. The chemical reaction of urea synthesis: 

Step 1: CO₂ + 2NH₃ → NH₂COONH₄ (ammonium carbamate) 
Step 2: NH₂COONH₄ → (NH₂)₂CO + H₂O 

The main materials treated in the urea production process are CO₂, NH₃, NH₂COONH₄, (NH₂)₂CO. When these materials exist alone, their corrosivity is not strong, but the reaction products produced by their mixing will cause strong corrosion to the equipment. The corrosion of urea-methyl ammonium solution to stainless steel in the urea production process is mainly caused by ammonium carbamate and ammonium cyanate in the solution. The main factors affecting corrosion resistance are the composition of the solution, the content of sulfide, the temperature, the relative velocity of the fluid, and the alloy element content and microstructure of the stainless steel material. The lining of the urea synthesis tower, stripping tower, and ammonium methylate condenser are easily corroded by ammonium methylate. In the 1950s, the Netherlands proposed to use 18-8 austenitic stainless steel under the condition of carbon dioxide and oxygen protection, but the effect could have been better. With the development and application of furnace refining technology, ultra-low carbon smelting of stainless steel has been realized, and the purity of steel has been improved. Finally, a series of urea-grade stainless steel materials have been formed to meet the requirements of urea production. Among them, the chemical composition, microstructure, and corrosion resistance of stainless steel are clearly defined. The purpose is to make stainless steel obtain full austenite structure and meet the requirements of lining and internals of urea high pressure equipment.

1.1 What is Urea Grade Stainless Steel?

Urea grade stainless steel is a special austenitic stainless steel. Urea grade stainless steel used for urea production mainly includes 316L UG, 310MOLN. Urea production is synthesized by carbon dioxide and ammonia under high pressure (14-25MPa) and a temperature of 180-210 degrees. The intermediate product ammonium carbamate has strong corrosion to stainless steel. Generally, stainless steel, such as 316L, cannot meet its corrosion resistance. The special steel clock developed is called urea grade stainless steel.

2. Technical requirements of urea grade stainless steel 

Specifically, urea grade stainless steel should meet the following requirements:

• 1) The chemical composition and mechanical properties of urea grade stainless steel should meet the national standard requirements of qualified urea equipment and materials.
• 2) The metallographic structure of the material is austenite, and the ferrite content is not more than 0.6%.
• 3) The material must pass the test, and the test results should meet the national standard requirements of urea equipment and materials.
• 4) The selective corrosion test should be carried out after the test, and the test results should meet the national standard requirements of urea equipment and materials.
• 5) Materials should also be metallographic examination, not contain Sigma phase, metal inclusions, and other requirements.

3. Characteristics of Urea Grade Stainless Steel

Urea-grade stainless steel is used in urea production equipment under high temperatures and high pressure in the highly corrosive medium of ammonium chloride solution, so the requirements are very strict.
1). Good weldability:
Generally speaking, urea grade stainless steel, like other austenitic stainless steels, has relatively good welding performance. However, due to the complex metallurgical process that is completed in a short period, the mechanical properties, metallographic structure, and corrosion resistance of welded joints (including deposited metal, fusion line, and heat affected zone) are always difficult to reach the base metal level. The welding seam is a weak link, and corrosion and damage to many pieces of equipment often occur around the welding seam. Therefore, the correct selection of welding materials and the formulation of reasonable welding processes are important issues in the manufacturing of urea equipment.
Urea grade stainless steel is pure austenitic stainless steel, prone to thermal cracks during welding. This type of crack belongs to crystalline cracks, which occur at high temperatures and no longer propagate at room temperature. The cause of hot cracking is due to the low thermal conductivity and high expansion coefficient of austenitic stainless steel, which prolongs the residence time of the weld metal in the high-temperature zone and increases the tensile strain experienced by the weld at high temperatures. In addition, there are often low melting point inclusions between the grains of pure austenitic welds, which exist as a liquid film between the austenite grains in the later stage of solidification crystallization. Under certain tensile stress, they crack and expand to form intergranular cracks. The elements that can form low melting point inclusions include S, P, etc. Many measures can be taken to prevent the occurrence of hot cracks in pure austenitic stainless steel welding, such as improving the Mn content in the weld seam, ω When Mn=4% -6%, it is quite effective in preventing hot cracks, strictly limiting the content of impurities such as S and P in the weld seam. Improving the cooling speed of the welding pool during welding and adopting corresponding measures in the process, such as short arc welding, low heat input, narrow pass technology, etc., to accelerate the pool’s cooling effectively prevent welding thermal cracks.
Urea-grade stainless steel is a single-phase austenitic structure. Still, within the range specified by the standard composition of the steel grade, there may be a small amount of ferrite due to differences in composition ratio and production process control. Due to the selective corrosion of ferrite in urea medium, urea grade stainless steel (including welding deposited metal) requires a ferrite content not exceeding 0.6%.
From the above analysis, it can be seen that there are two key control contents when welding urea grade stainless steel: ferrite content and welding hot cracking.
2). Excellent resistance to intergranular corrosion.
3). High resistance to pitting and crevice corrosion.

4. Types of urea grade stainless steel

The most common types of urea-grade stainless steel are:

310MoLN

The chemical composition of 310MoLN has been optimized for the specific use of the urea plant. It is a 310L modified austenitic stainless steel with low carbon, silicon, and high nitrogen added to stabilize and strengthen the austenitic phase. The alloy is specially designed to improve corrosion resistance in urea carbonate environments, including strippers. Due to the high content of chromium, molybdenum, and nitrogen (PREN > 33), this grade also has good corrosion resistance under wet corrosion conditions. UNS S31050 is designed to manufacture urea plants’ inner lining or supporting products (pipes, fittings). This grade can be used for urea stripper.

	Steel Grade
	Chemical Composition % of  1.4466, AISI 310MOLN, UNS S31050
	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  1.4465, AISI 310MOLN, UNS S31050

Tensile strength:	540-740	N/mm²
Yield strength 0,2% :	250	≥ N/mm²
Elongation:	35/30	≥ %
Hardness Brinell:	240	≤ HB

316L UG

UREA 316L grade is specially developed for the application of urea plants. It is 316L modified stainless steel with very low silicon and much higher molybdenum content. The low carbon content, coupled with a well-balanced chemical composition (low silicon and nickel content close to 14 %), makes the alloy completely austenitized without inter-metallic phase precipitation. Under solution annealing and water quenching, the ferrite level remains below 0.5 %. The UREA 316L grade is designed to manufacture the inner lining of urea plants to improve the corrosion resistance of urea-carbonate environments or supplemental products (pipes, fittings). The alloy is not designed for nitric acid applications.

Chemical Composition % of 1.4435, 316L UG, UNS S 31603

Grade		C	Si	Mn	P	S	Cr	Ni	Mo
316L UG	Min.	–	–	–	–	–	16	12.5	2.2
	Max.	0.03	0.4	2	0.045	0.03	19	15	3

Mechanical Properties of 1.4435, 316L UG, UNS S 31603

Tensile strength:	500-700	N/mm²
Yield strength 0,2% :	200	≥ N/mm²
Elongation:	40/30	≥ %
Hardness Brinell:	215	≤ HB

5. Production process of 316L Mod urea grade stainless steel plate

316L Mod is based on 316L austenitic stainless steel. By adjusting the content of Cr, Ni, Mo, and other elements and adopting a reasonable heat treatment process, the ideal austenite structure (ferrite content does not exceed 0.6%) and good resistance to grain boundary corrosion and selective corrosion are obtained.
Compared with ordinary 316L, 316L Mod stainless steel has more stringent requirements on both metallographic structure and corrosion resistance, which requires its composition design and heat treatment process control to be more stringent than ordinary 316L. 
The specific medium plate production process is: AOD smelting → continuous casting → slab grinding → slab heating → medium plate hot rolling → solution treatment performance test → surface pickling → packaging.
Control of main chemical components. In the alloy composition system of stainless steel, Cr can form Cr2O3 on the surface of stainless steel, which makes stainless steel have good corrosion resistance. The addition of Mo can make the oxide film of Cr dense and further improve the corrosion resistance of the material. Therefore, to improve the corrosion resistance of stainless steel, the actual control content of Cr and Mo in 316L Mod is higher than that of ordinary 316L. However, since both Cr and Mo are ferrite forming elements, their content should be controlled as much as possible to ensure that the upper limit is not generated.
In the urea production equipment, the content of harmful elements P and S is strictly controlled (P ≤ 0.030%, S ≤ 0.003%) due to the high performance requirements of the material to resist grain boundary corrosion, which is also necessary for the hot working plasticity control of the slab. The effect of P content on the corrosion resistance of sensitized 316L Mod urea grade stainless steel in the Huey test was studied. The results show that the P content in the steel reaches 0.06%, and the weight loss is 0%.

6. Application of Urea Grade Stainless Steel

Due to its excellent corrosion resistance, high strength, and high temperature performance, urea-grade stainless steel is widely used in the following fields:

• Fertilizer industry: Urea-grade stainless steel is widely used in the conveying system of fertilizer production equipment, which can transport urea, ammonia, and other media, as well as acidic or alkaline media.
• Petrochemical industry: urea-grade stainless steel is used in the transportation system in the petrochemical production process. The main media that can be transported are corrosive acid-base compounds and high-temperature rare gases.
• Pharmaceutical industry: urea-grade stainless steel is used in high-purity gas, water, chemicals, and other delivery systems, especially when high-purity water delivery systems need to use this pipe to ensure water purity.
• Food industry: Urea grade stainless steel is used to transport special media in the food industry, such as transporting melted chocolate and seaweed.
• Urea production industry: Urea grade stainless steel can transport highly corrosive media such as urea during urea production.

7. Hugh ‘s corrosion performance of urea grade 316L Mod stainless steel pipe

Urea grade 316L Mod stainless steel is a special material used in the manufacture of urea production equipment; it is an improved 316L steel in the chemical composition of nickel, chromium, molybdenum, and other elements of the mass fraction of certain adjustments, the purpose is to obtain better corrosion resistance than 316L steel. As urea production equipment is in high temperature, high pressure, and strongly corrosive (urea methyl ammonium liquid) environment, the equipment used in stainless steel will be strongly corroded. Therefore, corrosion problems often become the primary consideration of urea equipment design and manufacturing process. Hugh’s test method, that is, the non-sensitized state of boiling nitric acid method, and the corrosion state of urea equipment in the oxygen corrosion medium has a good consistency, can accurately reflect the corrosion resistance of 316L Mod steel in the urea production medium, so the test in the study of the corrosion resistance of steel for urea equipment has been widely used.
Nickel-chromium-molybdenum stainless steel manufacturing urea equipment in the operation process of the most harmful corrosion for the intergranular corrosion, and non-sensitized state of boiling nitric acid method is the solid state specimen placed in the density of 1.4g.cm^-3, the concentration of 15mol.L^-1 nitric acid solution boiled continuously for 48h, and then according to the specimen before and after the test of the difference in the quality of the test corrosion resistance of steel method. In the urea production medium and nitric acid solution medium caused by solid solution urea grade stainless steel intergranular corrosion is the main reason for the material in the impurity elements (phosphorus and silicon, etc.) in the grain boundary of the enrichment, resulting in the formation of crystal boundaries and intercrystalline corrosion in the electrolyte solution of the potential difference. The impact of different heat treatment processes on the urea grade 316L Mod stainless steel pipe Hugh’s corrosion performance needs to be studied more. Therefore, the author can test the material intergranular corrosion susceptibility of Hugh’s test method to evaluate the different solid solution heat treatment processes on the urea grade 316L Mod stainless steel pipe Hugh’s corrosion performance of the impact of the material in Hugh’s test process to observe and analyze the change of the corrosion morphology of the surface, and to study the urea grade 316L Mod stainless steel pipe Hugh’s corrosion performance of different heat treatment processes; the material is not a good choice. As well as the study of different heat treatment processes of urea grade 316L Mod stainless steel pipe after Hugh’s test surface corrosion morphology changes and its relationship with Hugh’s corrosion performance.

Test material and test method

The test material is ϕ38mm × 2.85mm urea-grade 316L Mod stainless steel cold rolled pipe. Its chemical composition is shown in Table 1, which can be seen to meet the “ASTM A182/A182M-2020 Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service” for urea grade 316L Mod stainless steel technical requirements.
Table.1 Chemical Composition of Urea Grade 316L Mod Stainless Steel (Mass Fraction)

Element	C	Mn	P	S	Si	Ni	Mo	Cr	N
Measured value	0.012	1.27	0.017	0.001	0.22	13.4	2.3	17.65	0.053
Standard value	≤0.030	≤2.00	≤0.045	≤0.030	≤1.00	10.00-15.00	2.00-3.00	16.00-18.00	≤0.100

Urea grade 316L Mod stainless steel cold rolled tube samples were put into the box furnace at 1000, 1025, 1050, 1075, 1100, 1125, and 1150 ℃ under the heat preservation for 30 min immediately after the water cooling, and then pickling to remove the surface of the specimen of the oxide skin. The specimens treated with different temperatures were cut into Hugh’s test specimens with a tube length of about 11 mm and a surface area of about 30 cm² by wire cutting, and then all surfaces of the specimens were ground step by step by 400-1000 mesh sandpaper, and the surface of the specimens was mechanically polished after the grinding, and then put into a nitric acid solution with a concentration of 15 mol.L^-1, and passed through the hydrogen fluoride and hot water for 10min of pickling, of which The volume ratio of nitric acid, hydrofluoric acid and hot water was 25\:4\:71, after which it was rinsed with water, scrubbed with a soft brush, and then blown dry and weighed, and then the dimensions were measured to calculate the surface area of the specimen, and then the surface area of the specimen was calculated in accordance with “ASTM A262-2015 Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels” technical requirements for Hugh’s test (boiling temperature of 98 ℃, boiling nitric acid test 48h for 1 cycle, a total of 5 cycles), calculate the corrosion rate of each cycle and the average corrosion rate. Using GX51 metallographic microscope to observe the surface corrosion morphology of the specimen in the process of Hugh’s test for each cycle, using S-3400N scanning electron microscope to observe the surface corrosion morphology of the specimen in the fifth cycle of Hugh’s test. The average grain size of the specimens after solid solution heat treatment at different temperatures was measured by the linear intercept method according to GB/T 6394-2017 “Method for Determining the Average Grain Size of Metal Materials”, and five fields of view were selected for each specimen to be measured. The ferrite content in the specimens was measured by an FMP30 ferrite tester.

Test results and discussion

Influence of heat treatment temperature on Hugh’s corrosion properties

Table 2 shows the corrosion rate of urea grade 316L Mod stainless steel pipe specimens under heat treatment at different temperatures. It can be seen that the cycle corrosion rate of each temperature specimen, with the increase in the corrosion cycle, has an overall trend of increasing. In contrast, with the increase in solid solution temperature, the average corrosion rate of the specimen is a downward trend. It shows that with the increase of the corrosion cycle of the material, corrosion aggravated, while the solid solution temperature is conducive to improving the material’s corrosion resistance.
Table.2 Corrosion rate of urea-grade 316L Mod stainless steel pipe specimens under different heat treatment processes

Heat treatment temperature/℃	Corrosion rate per cycle/(μm·d-1)					Average corrosion rate/(μm·d-1)
	1	2	3	4	5
1000	0.3233	0.5066	0.6466	0.8733	1.2133	0.7133
1025	0.3166	0.4733	0.5833	0.7533	0.8866	0.6033
1050	0.3500	0.4800	0.5600	0.6833	0.8666	0.5866
1075	0.3733	0.5733	0.6133	0.5900	0.9666	0.6233
1100	0.3800	0.5100	0.6666	0.4900	0.8666	0.5633
1125	0.2766	0.4266	0.4833	0.5233	0.6000	0.4633
1150	0.3366	0.4033	0.4433	0.5066	0.5466	0.4466

Corrosion morphology

Figure 1 shows the surface corrosion morphology of the specimens treated at 1000℃ for 30 min in different cycles during Hugh’s test. As seen in Figure 1a), some deep black corrosion pits appeared at the grain boundaries of the first cycle of the specimen, indicating that the first cycle of Hugh’s test has produced non-sensitized intergranular corrosion. Figure 1b) for the third cycle of the surface corrosion morphology of the specimen can be seen relative to the first cycle, the third cycle of the grain boundaries of deep black corrosion pits in the area of the further increase in the number of corrosion pits in the grain boundaries, indicating that with the increase in corrosion time intergranular corrosion along the grain boundaries to the depth of the development of the same time to the grain boundaries to the expansion of the sides, the corrosion pits cross-section morphology of the development of the V-shape, and more of the boundaries of the occurrence of intergranular corrosion. By Figure 1c), the fifth cycle of the sample surface corrosion morphology can be seen; some bright grains are surrounded by corrosion grooves all around, and some grains have been blurred, indicating that the fifth cycle of the sample intergranular corrosion is further aggravated. Figure 1 Hugh’s test process of different cycles of the evolution of the surface corrosion morphology of the specimen and Table 2 of the corrosion rate per cycle of comparative analysis shows that, with the increase of the test cycle, the non-sensitized state of the material intergranular corrosion degree of aggravation, and thus the cycle of the corrosion rate increases.

Figure.1 Surface corrosion morphology of 1000℃ heat preservation 30 min treated specimens in different cycles of Hugh’s test
Figure 2 shows the surface corrosion morphology of the specimens treated at 1075°C for 30 min during Hugh’s test in different cycles. It can be seen that the degree of intergranular corrosion becomes more and more serious with the increase of test cycles. Compared with Fig. 1, Fig. 2 shows a smaller number of deep black corrosion pits at the grain boundaries in the first cycle, and the total area of deep black corrosion pits at the grain boundaries in the third and fifth cycles is also smaller. Combined with the corresponding average corrosion rates in Table 2, it can be seen that the specimen treated at 1075°C produces less non-sensitized intergranular corrosion and better intergranular corrosion resistance than the specimen treated at 1000°C in the course of Hugh’s test and thus the average corrosion rate is also lower.

Figure.2 Surface corrosion morphology of the specimens treated at 1075℃ for 30 min in different cycles of Hugh’s test
Figure 3 shows the surface corrosion morphology of the specimens treated at 1150°C for 30 min during Hugh’s test in different cycles. It can be seen that there are no deep black corrosion pits at the grain boundaries of the first cycle specimens, there are several shallow, deep black corrosion pits at the grain boundaries of the third cycle specimens, and the intergranular corrosion of the fifth cycle specimens extends to both sides of the grain boundaries. The total area of corrosion pits further increases. The total area of deep black corrosion pits at the grain boundaries of the third and fifth cycle specimens in Fig. 3 is less than those of the third and fifth cycles in Fig.1 and Fig.2 combined with Table 2, it can be seen that the 1150°C treated specimens have better resistance to intergranular corrosion and Huey’s corrosion than the 1075°C treated and 1000°C-treated specimens.
From the above analysis, it can be seen that with the increase of the test cycle, the degree of intergranular corrosion of the material is aggravated; with the increase of the heat treatment temperature, the intergranular corrosion resistance and Hugh’s corrosion resistance of the material is improved.

Figure.3 Surface corrosion morphology of the specimens treated at 1150℃ for 30min in different cycles of the Hugh’s test
Figure.4 shows the scanning electron microscope (SEM) morphology of surface corrosion specimens treated with different temperatures of solid solution treatment in the fifth cycle of Hugh’s corrosion test. From Fig. 4a), it can be seen that there are a large number of relatively deep corrosion pits left on the surface of the specimen after the detachment of smaller sized grains, which is due to the penetration of corrosion inward along the grain boundaries until it surrounds a grain, resulting in the detachment of the grain. Visible from Figure 4b)-d), the organization is some relatively small grains at the grain boundaries of corrosion pits in the shape of a flared mouth. In contrast, the organization of several relatively large grains around the grain boundaries is surrounded by a narrow width of the corrosion groove. It indicates that the material is more prone to non-sensitized intergranular corrosion and more serious intergranular corrosion in Hugh’s test for small grain boundaries relative to large grain boundaries. This is because the grain boundaries of small grains are shorter and intergranular corrosion is more likely to penetrate along the grain boundaries, resulting in the detachment of small grains.
Figure.5 shows the average corrosion rate and grain size of the urea grade 316L Mod stainless steel pipe specimens Hugh’s test and the relationship between the solid solution temperature curve. It can be seen that as the heat treatment temperature increases, the average corrosion rate of the specimen is a decreasing trend. As the heat treatment temperature increases, the average grain size of the specimen gradually increases, and the average grain size and corrosion rate for the approximate inverse relationship. Due to the previous did not detect the urea grade 316L Mod stainless steel pipe intermediate by different temperature solution treatment of the presence of ferrite and heat treatment temperature in the austenitic single-phase zone, you can judge the average grain size to affect the average corrosion rate of urea grade 316L Mod stainless steel pipe specimens of different heat treatment process is the decisive factor. The larger the average grain size, the less grain is less likely to fall off; the better the material’s overall intergranular corrosion resistance, the lower the average corrosion rate of the specimen; that is, the material Hugh’s corrosion performance depends on the degree of intergranular corrosion, intergranular corrosion depends on the average grain size.

Fig.4 SEM morphology of surface corrosion of specimens’ heat treated by different processes in the fifth cycle of the Hugh’s test

Fig.5 Effect of heat treatment temperature on average corrosion rate and average grain size

Conclusion

• (1) with the increase of solid solution temperature, the average corrosion rate of urea grade 316L Mod stainless steel pipe decreases, and the resistance to Hugh’s corrosion increases. With the increase of Hugh’s corrosion test cycle, the corrosion rate of the material is on the rise, and Hugh’s corrosion is becoming more and more serious.
• (2) the average grain size determines the urea-grade 316L Mod stainless steel pipe resistance to non-sensitized intergranular corrosion performance and whether resistance to Hugh’s corrosion performance is good or bad. The average grain size is proportional to the solid solution temperature; the larger the size of the metal material, the better the intergranular corrosion resistance and the better Hugh’s corrosion resistance.

Author: Wang Hua