A method for toughening and strengthening a welded joint of an ultra-high-strength marine steel

Abstract

The application discloses a kind of super-high-strength marine steel welded joint toughening preparation methods, it is related to welding technical field.By coaxial powder feeding mode, FeMo55, Co and Cu micro-alloy powder are introduced into high-temperature area of molten pool synchronously, break through the limitation of traditional only relying on welding wire composition to control weld performance, realize the accurate control of weld metal composition and the optimization of organization, significantly improve the uniformity of organization and performance stability.Mo, Co and Cu elements introduced in the welding process produce synergistic strengthening effect, effectively control the solidification process and phase change behavior of weld, inhibit coarse structure and promote the formation of fine structure, and the prepared welded joint has excellent strength and toughness matching relationship, while maintaining good plasticity and low-temperature impact toughness, it meets the use requirements of high strength, high toughness and high reliability for marine structure equipment.

Description

Technical Field

[0001] This invention relates to the field of welding technology, specifically to a method for strengthening and toughening ultra-high strength marine steel welded joints. Background Technology

[0002] Marine engineering steel is a key material for constructing large marine equipment such as offshore platforms, ship hulls, and offshore wind turbine support structures. As marine engineering equipment develops towards larger sizes and deeper waters, the thickness and strength grades of marine engineering steel are continuously increasing, placing higher demands on the quality of welded joints. Gas metal arc welding (MIG / MAG welding) has become one of the most widely used welding methods in marine steel structure manufacturing due to its advantages such as high welding efficiency, high degree of automation, strong adaptability, and ease of engineering application. However, due to its uneven heat input distribution, large temperature gradient in the molten pool, and limited control over alloy composition, coarse columnar and dendritic crystals easily form during weld metal solidification, resulting in poor uniformity of the weld joint microstructure. This leads to a sharp decrease in the low-temperature impact toughness of the weld joint and increases its susceptibility to cold cracking.

[0003] Currently, the microstructure control of welded joints in ultra-high strength marine engineering steel mainly relies on the design of welding wire composition and oxide metallurgy techniques. Elements such as Cu, Ni, Mo, Mg, Cr, and rare earth elements Ce and La further improve the weld microstructure and mechanical properties through solid solution strengthening, precipitation strengthening, promoting nucleation, and regulating hardenability. However, existing technologies typically pre-add these elements to the welding wire or flux, which suffers from problems such as difficulty in flexibly controlling the addition amount, severe burn-off, and limited microstructure control effects. Therefore, developing a novel welding method that can precisely add alloying elements during the welding process, promote microstructure homogenization, and optimize performance is of great significance for improving the welding manufacturing quality and service reliability of marine engineering equipment.

[0004] Therefore, a method for strengthening and toughening ultra-high strength marine steel welded joints based on wire powder composite gas shielded welding is proposed to solve the above problems. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for strengthening and toughening ultra-high strength marine steel welded joints, comprising the following steps: Step 1: Process the marine steel base material to be welded into a Y-shaped bevel and perform pre-welding treatment; Step 2: Select ER120S-G solid welding wire and prepare alloy metal powder; Step 3: Use wire-powder composite gas metal arc welding. Set up a coaxial wire feeding system and a powder feeding system to continuously feed solid welding wire, while simultaneously feeding alloy metal powder into the high-temperature zone of the molten pool through a powder feeding nozzle. Control the total powder feeding rate to be 8-14 g / min, the wire feeding speed to be 7-9 m / min, the welding wire extension to be 14-18 mm, the welding current to be 240-280 A, the welding voltage to be 26-30 V, the welding speed to be 5-7 mm / s, the shielding gas to be Ar, and the shielding gas flow rate to be 5 L / min. Step 4: Allow the weld to cool naturally to room temperature after welding, then clean the weld seam.

[0006] Preferably, the alloy metal powder comprises one or more of FeMo55 powder, pure Co powder, and pure Cu powder.

[0007] Preferably, the FeMo55 powder contains 55% to 60% Mo by mass, with the remainder being Fe and impurities; the pure Co powder has a purity ≥99.5%; and the pure Cu powder has a purity ≥99.5%.

[0008] Preferably, the alloy metal powder is prepared by mechanical ball milling. The powder is weighed according to the designed ratio and placed in a planetary ball mill. Under argon protection, it is ball milled at 300 r / min for 4 hours to mix evenly. The powder is then dried and sieved for later use. The particle size range of the alloy metal powder is 20–60 μm.

[0009] Preferably, in step 3, the distance between the powder spraying point and the center of the electric arc is 0-10 mm, and the distance between the conductive tip of the wire feeder and the surface of the marine steel base material to be welded is 20-25 mm; the powder feeding gas is Ar, and the carrier gas flow rate is 20 L / min.

[0010] Preferably, the pre-welding pretreatment in step 1 is as follows: Grind both sides of the Y-shaped bevel, and clean the bevel and both sides with acetone to remove oil and dust.

[0011] Preferably, the bevel angle of the Y-shaped bevel is 60° and the distance between the bottom of the Y-shaped bevel is 2mm.

[0012] Preferably, the solid welding wire has a diameter of 1.2 mm and a chemical composition by mass percentage as follows: The composition is as follows: C 0.10%–0.13%, Si 0.50%–0.80%, Mn 1.60%–2.10%, Ni 2.30%–2.80%, Mo 0.30%–0.65%, Al 0%–0.30%, P 0%–0.008%, S 0%–0.008%, with the balance being Fe and impurities.

[0013] The present invention has the following beneficial effects: This invention employs wire-powder composite gas shielded welding technology, simultaneously introducing FeMo55, Co, and Cu microalloyed powders into the high-temperature region of the molten pool via coaxial powder feeding. This overcomes the limitations of traditional methods that rely solely on welding wire composition to control weld performance, achieving precise control and microstructure optimization of the weld metal composition, significantly improving microstructure uniformity and performance stability. The introduced Mo, Co, and Cu elements exert a synergistic strengthening effect during welding, effectively regulating the weld solidification process and phase transformation behavior, suppressing coarse microstructures and promoting fine microstructure formation. The resulting welded joint exhibits an excellent strength-toughness balance, maintaining good plasticity and low-temperature impact toughness while achieving high yield strength and tensile strength, meeting the high strength, high toughness, and high reliability requirements of marine structural equipment.

[0014] This invention achieves the technical effects of controllable weld metal composition, optimized microstructure, and improved performance by quantitatively adding microalloy powder. Under the premise of simple process and controllable cost, it significantly improves the strength, toughness and comprehensive service performance of ultra-high strength marine steel welded joints. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the welding bevel of the present invention; Figure 2 This is a schematic diagram of the coaxial powder feeding structure in this invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0018] Example 1: refer to Figures 1 to 2 This embodiment provides a method for strengthening and toughening ultra-high strength marine steel welded joints, including the following steps: Step 1: Process the marine steel base material to be welded into a Y-shaped bevel and perform pre-welding treatment (grind within 40mm on both sides of the bevel to a metallic luster, and clean the bevel and both sides with acetone to remove oil and dust). Step 2: Select ER120S-G solid welding wire and prepare alloy metal powder; Step 3: Using wire-powder composite gas metal arc welding, the existing coaxial wire feeding system and powder feeding system are set up. During the welding process, the welding wire is fed into the molten pool through the center of the welding torch, and the alloy metal powder is continuously transported to the arc zone through the coaxial powder feeding system. Under the action of the arc heat source, it melts synchronously with the welding wire and the base metal. The total powder feeding rate is controlled at 12g / min, the wire feeding speed is 8m / min, the welding wire extension is controlled at 16mm, the welding current is 260A, the welding voltage is 28V, the welding speed is 6mm / s, the powder feeding gas is Ar, the carrier gas flow rate is 20L / min, the shielding gas is Ar, and the shielding gas flow rate is 5L / min. Step 4: After welding, allow the weld to cool naturally to room temperature. After cooling, remove the oxide scale and other deposits from the weld surface and both sides to expose the metallic luster.

[0019] In this embodiment, a 10mm thick marine engineering steel plate is selected as the base material, with plate dimensions of 150mm × 60mm × 10mm. The chemical composition of this marine engineering steel, by mass percentage, is: C: 0.02%–0.07%, Si: 0.10%–0.25%, Mn: 1.75%–2.5%, Ni: 12%–16%, Mo: 1%–1.5%, Cr: 3.5%–4%, Co: 15%–18%, Cu: 0.65%–0.68%, V: 0.12%–0.16%; Ti: 0.045%–0.055%, Sn: 0.03%–0.05%, Sb: 0.03%–0.05%, Re: 0.03%–0.05%, P: 0.026%–0.022%, S: 0%–0.08%, with the balance being Fe and unavoidable impurities.

[0020] The base material has a yield strength of not less than 1200 MPa, a tensile strength of not less than 1300 MPa, and an elongation after fracture of not less than 14%.

[0021] In this embodiment, the bevel angle of the Y-shaped groove is 60°, the blunt edge is 1.5mm, and the root gap is 2mm; the solid welding wire used is ER120S-G solid welding wire with a diameter of Φ1.2mm. The chemical composition of the welding wire by mass percentage is: C: 0.10%~0.13%, Si: 0.50~0.80%, Mn: 1.60%~2.10%, Ni: 2.30%~2.80%, Mo: 0.30%~0.65%, Al: 0%~0.30%, P: 0%~0.008%, S: 0%~0.008%, with the balance being Fe and unavoidable impurities.

[0022] In this embodiment, the alloy metal powder includes reinforcing elements molybdenum, cobalt, and copper, with a Mo:Co:Cu mass ratio of 0.5:8:0.5. Specifically, the molybdenum element is ferromolybdenum (FeMo55) powder, in which the Mo mass content is 55%–60%, with the remainder being Fe and unavoidable impurities; the cobalt element is pure Co powder with a purity ≥99.5%; and the copper element is pure Cu powder with a purity ≥99.5%. Since the Mo element is provided using ferromolybdenum (FeMo55) powder, the mass percentages of ferromolybdenum (FeMo55) powder, cobalt (Co) powder, and copper (Cu) powder are calculated using the following formula: ; ; ; In the formula, This represents the mass percentage of FeMo55 powder in the alloy powder. This represents the mass percentage of Co powder in the alloy powder. This represents the mass percentage of Cu powder in the alloy powder. The mass fraction (dimensionless) of Mo in the alloy powder according to the target ratio of strengthening elements. The mass fraction (dimensionless) of Co in the alloy powder according to the target ratio of strengthening elements. The mass fraction (dimensionless) of Cu in the alloy powder according to the target ratio of strengthening elements. The mass fraction of Mo in ferromolybdenum (FeMo55) is dimensionless.

[0023] The powder composition, calculated by mass percentage, is 9.7% FeMo55 powder, 85% Co powder, and 5.3% Cu powder.

[0024] In this embodiment, the absolute powder feeding rates of ferromolybdenum (FeMo55) powder, cobalt (Co) powder, and copper (Cu) powder are calculated using the following formula: ; In the formula, For the first The absolute powder feeding rate, expressed in g / min. This represents the total powder feeding rate, expressed in g / min. For the first The mass percentage of the powder.

[0025] At a total powder feeding rate of 12 g / min, the absolute powder feeding rate of ferromolybdenum (FeMo55) powder is 1.2 g / min, the absolute powder feeding rate of cobalt (Co) powder is 10.2 g / min, and the absolute powder feeding rate of copper (Cu) powder is 0.6 g / min.

[0026] In this embodiment, the alloy metal powder was prepared by mechanical ball milling. The powder was weighed according to the design ratio and placed in a planetary ball mill. Under argon protection, it was ball milled at 300 r / min for 4 hours to mix evenly. The powder was then dried and sieved for later use. The particle size range of the alloy metal powder was 20–60 μm.

[0027] Example 2: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The composition ratio of the alloy metal powder was adjusted. The mass ratio of the reinforcing elements Mo:Co:Cu in the alloy metal powder was 1:8:0.5. The powder composition, calculated by mass percentage, was 17.6% for FeMo55 powder, 77.5% for Co powder, and 4.9% for Cu powder. The absolute feed rate of ferromolybdenum (FeMo55) powder was increased, while the absolute feed rates of cobalt (Co) powder and copper (Cu) powder remained the same as in Example 1. In this example, the total feed rate was adjusted to 13 g / min. The absolute feed rates of each component powder were calculated according to the formula. The absolute feed rate of cobalt (Co) powder was 10.2 g / min, and the absolute feed rate of copper (Cu) powder was 0.6 g / min, which was basically the same as in Example 1. The absolute feed rate of ferromolybdenum (FeMo55) powder was increased from 1.2 g / min in Example 1 to 2.2 g / min.

[0028] The remaining steps are the same as in Example 1.

[0029] Example 3: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The composition ratio of the alloy metal powder was adjusted. The mass ratio of the reinforcing elements Mo:Co:Cu in the alloy metal powder was 0.5:8:1. The powder composition, calculated by mass percentage, was 9.2% for FeMo55 powder, 80.7% for Co powder, and 10.1% for Cu powder. The absolute feed rate of copper (Cu) powder was increased while maintaining the same absolute feed rates of ferromolybdenum (FeMo55) powder and cobalt (Co) powder as in Example 1. In this example, the total feed rate was adjusted to 12.6 g / min. The absolute feed rates of each component powder were calculated according to the formula. The absolute feed rate of ferromolybdenum (FeMo55) powder was 1.2 g / min, and the absolute feed rate of cobalt (Co) powder was 10.2 g / min, which is basically the same as in Example 1. The absolute feed rate of copper (Cu) powder was increased from 0.6 g / min in Example 1 to 1.2 g / min.

[0030] The remaining steps are the same as in Example 1.

[0031] Example 4: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The composition ratio of the alloy metal powder was adjusted. The mass ratio of the reinforcing elements Mo:Co:Cu in the alloy metal powder was 0:8:0.5. The powder composition, calculated by mass percentage, was 94.1% Co powder and 5.9% Cu powder. The alloy powder does not contain ferromolybdenum (FeMo55) powder. The absolute powder feeding rates of cobalt (Co) powder and copper (Cu) powder are kept the same as in Example 1. In this example, the total powder feeding rate is adjusted to 10.8 g / min. The absolute powder feeding rates of each component powder are calculated according to the formula. The absolute powder feeding rate of cobalt (Co) powder is 10.2 g / min, and the absolute powder feeding rate of copper (Cu) powder is 0.6 g / min, which is basically the same as in Example 1. The absolute powder feeding rate of ferromolybdenum (FeMo55) powder is reduced from 1.2 g / min in Example 1 to 0 g / min.

[0032] The remaining steps are the same as in Example 1.

[0033] Example 5: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The composition ratio of the alloy metal powder was adjusted. The mass ratio of the reinforcing elements Mo:Co:Cu in the alloy metal powder was 0.5:0:0.5. The powder composition, calculated by mass percentage, was 64.5% for FeMo55 powder and 35.5% for Cu powder. The alloy powder does not contain cobalt (Co) powder. The absolute powder feeding rates of ferromolybdenum (FeMo55) powder and copper (Cu) powder are kept the same as in Example 1. In this example, the total powder feeding rate is adjusted to 1.8 g / min. The absolute powder feeding rates of each component powder are calculated according to the formula. The absolute powder feeding rate of ferromolybdenum (FeMo55) powder is 1.2 g / min, and the absolute powder feeding rate of copper (Cu) powder is 0.6 g / min, which is basically the same as in Example 1. The absolute powder feeding rate of cobalt (Co) powder is reduced from 10.2 g / min in Example 1 to 0 g / min.

[0034] The remaining steps are the same as in Example 1.

[0035] Example 6: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The composition ratio of the alloy metal powder was adjusted. The mass ratio of the reinforcing elements Mo:Co:Cu in the alloy metal powder was 0.5:8:0. The powder composition, calculated as a mass percentage, was 10.2% for FeMo55 powder and 89.8% for Co powder. The alloy powder does not contain copper (Cu) powder. The absolute powder feeding rates of ferromolybdenum (FeMo55) powder and cobalt (Co) powder are kept the same as in Example 1. In this example, the total powder feeding rate is adjusted to 11.4 g / min. The absolute powder feeding rates of each component powder are calculated according to the formula. The absolute powder feeding rate of ferromolybdenum (FeMo55) powder is 1.2 g / min, and the absolute powder feeding rate of cobalt (Co) powder is 10.2 g / min, which is basically the same as in Example 1. The absolute powder feeding rate of copper (Cu) powder is reduced from 0.6 g / min in Example 1 to 0 g / min.

[0036] The remaining steps are the same as in Example 1.

[0037] Example 7: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The total powder feeding rate is set to 8g / min.

[0038] The remaining steps are the same as in Example 1.

[0039] Example 8: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The total powder feeding rate is set to 10g / min.

[0040] The remaining steps are the same as in Example 1.

[0041] Example 9: This embodiment uses the same base material, welding wire, powder type, and pretreatment process as Example 1. The difference from Example 1 is that: The total powder feeding rate is set to 14g / min.

[0042] The remaining steps are the same as in Example 1.

[0043] Comparative Example 1: This embodiment uses the same base material, welding wire, and pretreatment process as in Embodiment 1, but does not add alloy metal powder and adopts conventional pulsed MIG welding.

[0044] The performance test results of the welded joints in Examples 1 to 9 and Comparative Example 1 are shown in Table 1: (Table 1) As shown in Table 1, the yield strength of the welded joint in Comparative Example 1 is 965 MPa, the tensile strength is 1010 MPa, and the elongation after fracture is 14.4%. The impact energy at 40℃ is 50J. After adopting the wire-powder composite welding process of this invention, in terms of strength, the yield strength of the welded joints in Examples 1-9 reaches 988-1367MPa, and the tensile strength reaches 1046-1430MPa, with an overall strength level significantly higher than Comparative Example 1. Regarding plasticity, the elongation after fracture in Example 3 reaches 17.2%, an increase of approximately 19.4% compared to Comparative Example 1; the elongation after fracture in Example 7 reaches 16.5%, an increase of approximately 14.6%; and the elongation after fracture in Example 9 reaches 15.9%, an increase of approximately 10.4%. In terms of low-temperature impact toughness, Example 3 reaches 66J, an increase of 32.0% compared to Comparative Example 1; Example 7 reaches the highest value of 68J, an increase of 36.0%; and Example 9 reaches 62J, an increase of 24.0%.

[0045] Mo (Mo) exhibits strong solid solution strengthening properties, improving the hardenability of the matrix and promoting the formation of carbides and alloying strengthening phases. Co (Co), when dissolved in the iron matrix, enhances lattice distortion, increases dislocation slip resistance, and promotes microstructure refinement. Simultaneously, Co promotes dispersed carbide precipitation, improving the thermal stability of the microstructure. Cu (Cu) further enhances material strength through solid solution strengthening and precipitation strengthening. The synergistic effect of these three elements optimizes the weld microstructure, improving the strength and toughness of the welded joint. This demonstrates that rationally controlling the proportions of FeMo55, Co, and Cu can improve weld strength while maintaining good plasticity and low-temperature impact toughness, achieving a synergistic improvement in the strength, plasticity, and toughness of the welded joint. This meets the application requirements of marine engineering equipment for high-strength, high-toughness, and high-reliability welded joints.

[0046] In summary, this invention employs wire-powder composite gas shielded welding technology, simultaneously introducing FeMo55, Co, and Cu microalloyed powders into the high-temperature region of the molten pool via coaxial powder feeding. This overcomes the limitations of traditional methods that rely solely on the welding wire composition to control weld performance, achieving precise control and microstructure optimization of the weld metal composition, significantly improving microstructure uniformity and performance stability. The introduced Mo, Co, and Cu elements exert a synergistic strengthening effect during welding, effectively regulating the weld solidification process and phase transformation behavior, suppressing coarse microstructures, and promoting the formation of fine microstructures. The resulting welded joint exhibits an excellent strength-toughness balance, maintaining good plasticity and low-temperature impact toughness while achieving high yield strength and tensile strength, meeting the high strength, high toughness, and high reliability requirements of marine structural equipment. By quantitatively adding microalloying powder, the technical effects of controllable weld metal composition, optimized microstructure, and improved performance were achieved. Under the premise of simple process and controllable cost, the strength, toughness and comprehensive service performance of ultra-high strength marine steel welded joints were significantly improved.

[0047] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for strengthening and toughening ultra-high strength marine steel welded joints, characterized in that, Includes the following steps: Step 1: Process the marine steel base material to be welded into a Y-shaped bevel and perform pre-welding treatment; Step 2: Select ER120S-G solid welding wire and prepare alloy metal powder; Step 3: Use wire-powder composite gas metal arc welding. Set up a coaxial wire feeding system and a powder feeding system to continuously feed solid welding wire, while simultaneously feeding alloy metal powder into the high-temperature zone of the molten pool through a powder feeding nozzle. Control the total powder feeding rate to be 8-14 g / min, the wire feeding speed to be 7-9 m / min, the welding wire extension to be 14-18 mm, the welding current to be 240-280 A, the welding voltage to be 26-30 V, the welding speed to be 5-7 mm / s, the shielding gas to be Ar, and the shielding gas flow rate to be 5 L / min. Step 4: Allow the weld to cool naturally to room temperature after welding, then clean the weld seam.

2. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 1, characterized in that, The alloy metal powder includes one or more of FeMo55 powder, pure Co powder, and pure Cu powder.

3. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 2, characterized in that, The mass content of Mo in FeMo55 powder is 55% to 60%, with the remainder being Fe and impurities; the purity of pure Co powder is ≥99.5%; the purity of pure Cu powder is ≥99.5%.

4. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 2, characterized in that, The alloy metal powder was prepared by mechanical ball milling. The powder was weighed according to the design ratio and placed in a planetary ball mill. Under argon protection, it was ball milled at 300 r / min for 4 hours to mix evenly. The powder was then dried and sieved for later use. The particle size range of the alloy metal powder was 20–60 μm.

5. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 1, characterized in that, In step 3, the distance between the powder spraying point and the center of the electric arc is 0-10 mm, and the distance between the conductive tip of the wire feeder and the surface of the marine steel base material to be welded is 20-25 mm; the powder feeding gas is Ar, and the carrier gas flow rate is 20 L / min.

6. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 1, characterized in that, The pre-welding pretreatment in step 1 is as follows: Grind both sides of the Y-shaped bevel, and clean the bevel and both sides with acetone to remove oil and dust.

7. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 1, characterized in that, The bevel angle of the Y-shaped bevel is 60°, and the distance between the bottom of the Y-shaped bevel is 2mm.

8. The method for strengthening and toughening an ultra-high strength marine steel welded joint according to claim 1, characterized in that, The solid welding wire has a diameter of 1.2mm and its chemical composition by mass percentage is: The composition is as follows: C 0.10%–0.13%, Si 0.50%–0.80%, Mn 1.60%–2.10%, Ni 2.30%–2.80%, Mo 0.30%–0.65%, Al 0%–0.30%, P 0%–0.008%, S 0%–0.008%, with the balance being Fe and impurities.

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