Structural and mechanical characterization of S355J2 weldable

The 6th International Conference – Innovative technologies for joining advanced materials

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Structural and mechanical characterization of S355J2 weldable steel from the plan safety gate construction -

PdF1 Dr Tr Severin D. R. Pascu1, M. Vlaia2, R. A. Roşu1, N. A. Sîrbu1

1 National R&D Institute for Welding and Material Testing, 30 Mihai Viteazul Bv, 300222 Timisoara, Romania 2 HIDROTIM Timişoara

E-mail: [email protected]

Abstract

Plan safety gate - PdF1 with the length of 12,370 mm and the width of 3300 mm is part of the safety system from PdF1 Drobeta Tr. Severin, being covered with a protective coating resistant to water from Danube. Non-alloyed steel used to build the gate is from C-Mn class, mark OL52-4kf (S355J2) that has guaranteed the breaking energy of 27J at the tests temperatures of +20°C and -20°C.

Evaluation of the structural and mechanical characteristics of the non-alloyed steel with the thickness of 12 and 40mm was performed on six cores samples with Ø250mm taken three from the upper left bank (to Romania) and three symmetrically in the lower right bank (to Serbia).

The structures of the examined steel are ferrite-pearlite with mechanical resistance Rm of min. 531 N/mm2 and Rp0,2 of min. 385 N/mm2 and fracture energies at +20°C and at -20°C of min 53J that are above the values for S355J2 steel specified by the norm SR EN 10025:94.

Aging phenomena of the steel are accentuated at the hidrotechnic component structure; in this actual state at -20°C the breaking energy values are low, below the 27J, considered the minimum required value.

Repairing technology by welding of the analyzed component respecting the technical solutions to restore the local structures from where were taken the cores for experiments require qualification of the welding procedures according to current European standards.

1. Introduction

Plan safety gate which equips the upstream head of Romanian part from SHEN Porţile de Fier I and provide the lockage of the ships since 1969, was designed by SCHIDROTIM SA Timisoara.

The gate is a metal construction part made of unalloyed steel sheets rolled, joined by welding, equipped with sealing systems, with a chassis and wheels guide/rolling gate that provides translation by a hydraulic system, to close/open the gate.

Included in the program of refurbishment, after over forty years of operation, the gate was included in an extensive program of expertise in which they were prescribed non-destructive testing performed on samples of metal taken from the steel structure according with the plan drawing V2239-71. The plan highlights areas where the steel samples were taken and technical solutions to local recovery the structure after sampling.

Destructive tests, performed by ISIM Timisoara aimed to determine mechanical, technological and toughness as well as determining the aging resistance of the steel structure.

2. Materials presented Beneficiary SC HIDROTIM - Timisoara presented at ISIM Timisoara 6 samples Ø250 mm namely:

• 3 samples of the upper section on the left bank (to Romania) marked with SS1, SS2 and SS3.

• 3 samples of the lower section right bank (to Serbia) marked SI1, SI2, SI3

On each sample was marked direction of Danube water flow. Metallic material used to realize the components was non-alloyed steel S355J2 according to SR EN 10025:1994.

3. Experimental program The experimental program included a series of chemical analysis, structural examinations and mechanical tests to characterize the non-alloyed steel from the construction of Plan safety gate from PdF1 Dr Tr Severin namely:

• Visual analysis of cylindrical cores

• The chemical composition of the steels

• Metallographic microstructures of the samples, HV10 hardness and local hardening estimator

• The mechanical characteristics of strength, deformation and toughness of steel

• Analysis of the aging phenomena


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3.1 Analysis of cylindrical cores visual Visual analysis of cylindrical cores taken from the mechanical components highlighted:

• In all samples, corrosion protection was destroyed on large areas of the sample at a rate of max. 80% (Figures 1 ... 6).

• On the core sample surfaces was not observed cracks or other defects.

Figure 1. Core sample SS Figure 2. Core sample SS2

Figure 3. Core sample SS3 Figure 4. Core sample SI1

Figure 5. Core sample SI2 Figure 6. Core sample SI3


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3.2 Chemical characterization

The variations of the main chemical elements (C, Si, Mn) from S355J2G2 steel with the thickness of 20mm and 40 mm are shown in Figures 7 and 8.

Analyzing the variations of the main chemical elements it is observing that that examined steels (thickness of 40 mm and 20 mm) has a high chemical homogeneity, all percentage values of major chemical elements are placed in the concentrates required by EN 10025:94 for the brand analyzed steel.

TABLE 1. CHEMICAL COMPOSITION

Chemical composition [%] C* Mn Si Cr V

Sample SS1 (M1) (40mm thickness) 0.21 1.12 0.25 0.13 0.08 0.20 1.17 0.26 0.12 0.07 0.19 1.16 0.26 0.11 0.06



Sample SSI (M4) (40mm thickness) 0.20 1.17 0.27 0.09 0.08 0.20 1.25 0.29 0.11 0.08 0.18 1.22 0.27 0.10 0.09



S355J2G2 Steel SR EN 10025:94 Max 0.23 Max 1.70 Max 0.60 - -

*Carbon determined with Quantodesk device from UPT Timişoara

Figure 7. Variations of the chemical elements at S355J2G2 steel, thickness 20mm


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Figure 8. Variations of the chemical elements at S355J2G2 steel, thickness 40mm

3.3 Structural characterization

3.3.1 Macroscopic examination

Macroscopic examination of metallographic samples (M1 ... M6) Ø250mm cores presented not manufacturing defects both on longitudinal and transverse sections.

3.3.2 Microscopic examinations

Microscopic examinations performed according to SR 9000-97 and STAS 550-74 on optical microscope MFe2 from ISIM Timisoara evidenced at samples M1 ... M6 in longitudinal sections normal ferrito-pearlitic microstructures in rows, with the grain score 8-9 according to SR ISO 643:2003 (Figures 9 ... 14).

Figure 9. Sample M1 [Etching Nital 2%, 100x] Figure 10. Sample M2 [Etching Nital 2%, 100x]



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In all cases in microstructures were observed nonmetallic inclusions such as sulfides (MnS), evidencing a low purity of unalloyed steels examined in longitudinal direction (rolling direction).

Ferrite fine-pearlite microstructures in rows show that the manufacture process of steel plate was hot plastic deformation (rolling) where the cooling rate after deformation was not controlled continuous [1].

On the metallographic examination of the samples surface were not detected manufacturing defects such as microcracks.

3.4 Mechanical Characterization

3.4.1 HV10hardness tests

HV10 hardness tests according to EN ISO 6507-1:2006 using Zwick 3212 device were performed by traces on the thickness of alloy steels in longitudinal section (L) and transverse (T) according to Figure 15.

In table 2 were introduced minimum and maximum HV10 hardness values and the local hardening estimator values ΔHV10 calculated with the relation 1.

ΔHV10= [%]100HV10

HV10HV10

max

minmax ⋅− (1)

where:

• HV10max is the maximum HV10 hardness in one zone of the steel;

• HV10min is the minimum HV10 hardness in another zone of the steel

It is considered that if ΔHV1 ≥ 50% in the analyzed areas were developed hardening-embrittlement structural phenomena with high risk of producing brittle-type fracture [2].

TABLE 2 . HV10 HARDNESS

Sample Examined section

HV10 hardness Local hardening estimator, ΔHV10 Val.

min

Val

max.

M1 (SS1) L 176 181 2,76

T 161 182 11,53

M2 (SS2) L 164 168 2,38

T 169 175 3,42

M3 (SS3) L 167 172 2,90

T 170 175 2,85

M4 (SI1) L 162 170 4,70

T 160 198 19,19

M5 (SI2) L 161 165 2,42

T 161 169 4,73

M6 (SI3) L 152 163 6,74

T 155 157 1,27

BMT BML

Figure 15. Hardness measurement scheme


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The variation of local hardening estimator ΔHV10 at maximum values of the analyzed samples is presented in figure 16.

Analyzing ΔHV10 estimator to the maximum values for the analyzed samples it is observing that it varies between 2.90 and 19.19%, values that are below 50%, attesting that the risk of local hardening phenomena is reduced and thus there is a decreased tendency of brittle-type fracture.

The maximum values of local hardening ΔHV10 estimator (11.53% and 19.19%) are found in samples M1 and M4 from the steel with the thickness of 40mm.

3.4.2 Tensile testing

Tensile testing performed on cylindrical samples taken from the six core respecting the norm SR EN 895:1997 revealed the minimum and maximum mechanical strength characteristics (Rp0,2, Rm) and the deformation (A5, Z) that are listed in Table 3. Also in Table 3 are presented and the values imposed by SR EN 10025:94 for the mechanical characteristics of S355J2G2 steel.

In Figures 17 and 18 are presented the variation of mechanical characteristics (Rp0,2, Rm, A5, Z) for S355J2G2 steels thickness 20 mm (core sample SS2) and 40 mm (core sample SS1) left bank of the upper areas (to Romania).

Figure 16. ΔHV10 variation

TABLE 3. MECHANICAL CHARACTERISTICS

Sample

Mechanical characteristics

Rp0,2 (N/mm2) Rm (N/mm2) A5[%] Z [%]

val. min val max. val. min val max. val. min val max. val. min val max.

SS1 389 416 528 531 31 32 68 71

SS2 405 422 530 534 31 32 69 70

SS3 409 417 539 540 32 32 71 72

SI1 403 403 540 542 32 33 68 68

SI2 405 410 533 534 34 34 71 71

SI3 389 376 514 515 35 35 65 67

S355J2G2 steel SR EN 10025:94 340 - 510 630 22 - - -

Figure 17. Rp0,2 , Rm, A5, Z variation at core sample SS2 (thickness 20mm)


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Figure 18. Rp0,2 , Rm, A5, Z variation at core sample SS2 (thickness 40mm)

Analyzing the values of mechanical strength and deformability obtained shows that the values fall within the provisions required by the standard SR EN 10025:94 for unalloyed steel S355J2G2 mark.

3.4.3 Impact bend test

Impact bending test according to EN 895:1997 on samples with "V" channel was performed at room temperature and at -20°C, the values of breaking energies KV0 are presented in Table 4. Also in Table 4 were inserted and breaking energy values (at 20°C and -20°C) required for non-alloy steel S355J2G2 by the norm EN 10025:94.

Analyzing the breaking energy values KV0 (current state) it is observing that this characteristic has the values of min. 62J and max. 109J, values which are more than the value required (27J) (at both test temperatures) the surfaces fracture aspect being ductile, for which in the current state of the steels have high toughness at a temperature of 20°C and at a temperature of -20°C.

3.4.4 Analysis of aging phenomena

Aging tendency [3] of the steels was assessed by measuring the fracture energy KVI on resilience specimens plastically deformed with the degree of 7% and then subjected to a heat treatment at 250°C for one hour according to STAS 56774:79. The experimental results are given in Table 5 where there area also presented and the aging degree GI values calculated with relation (2):

GI= [%]1000

0 ⋅−

KVKVKV I (2)

where: • KV0 is the maximum value of the breaking energy

determined in actual state (0) • KVI is the minimum value of the breaking energy

determined in aged state (I)

It is considered that the analyzed steel shows a pronounced sensitivity to aging if GI ≥ 50%.

TABLE 4. BREAKING ENERGY

Sample mark/thickness Breaking energy, KV0 [J] Test temperature +20°C Test temperature -20°C

Minimum value Maximum value Minimum value Maximum value SS1 (40mm) 97 99 86 90 SS2 (20mm) 104 111 103 109 SS3 (20mm) 91 95 88 94 SI1(40mm) 92 99 90 93 SI2 (20mm) 104 105 104 107 SI3 (20mm) 62 64 63 64

S355J2G2 steel SR EN 10025:94

27 - 27 -

TABLE 5 BREAKING ENERGY

Sample mark/thickness

Breaking energy KVI [J] Aging degree, GI [%] Test temperature +20°C, Test temperature -20°C

Min val. Max. val. Min val. Max. val. la +20°C

la -20°C

SS1 (40mm) 44 79 11 23 55,5 87,8 SS2 (20mm) 80 95 10 15 27,9 90,8 SS3 (20mm) 58 90 19 26 38,9 79,8 SI1(40mm) 53 60 18 23 46,5 80,6 SI2 (20mm) 47 81 12 21 55,5 88,8 SI3 (20mm) 49 52 14 17 22,2 78,1


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Variation of the aging degree GI of the steels with the thickness of 40 mm and 20 mm (SS1 and SS3 cores) and the core samples SI1 and SI2 is presented in Figure 19.

Figure 19. GI = f(sample core) variation

Analyzing the aging degree variation GI of analyzed steels with the thickness of 20 and 40 mm is observed that most of the values are higher than 50% except for the values from sample SS3 (at 20°C) and SI1 (at 20°C), which shows a high predisposition to aging at the temperature of -20°C (GI varies between 80.5 and 98.8%) [4].

4. Local recovery solution structure Local recovery solution structure of sampling cores (samples) includes:

- welding of reinforcement washers, - new parts replace-position samples, - welding execution according with SR 1911:98, - local protection recovery.

Model for achieving local restoration by welding for the thicknesses of 20 and 40 mm is shown in Figure 20.

Figure 20. Model for achieving local restoration by welding

After welding of the cut areas will be made 100% control on the welds lengths and in the absence of the defects will starting to prepare technical documents for qualification of welding technologies under the current rules.

4. Conclusions 5.1 Wear safety plan is part of the upstream head of Romanian and provide ships lockage on the Danube at the Iron Gates I (PdF1)

5.2 The materials with the thickness of 20 and 40 mm subjected to expertise belongs to C-Mn group steel alloy which does not exceed the carbon percentage of 0.21% and manganese of 1% chemical compositions which fit the steel mark S355J2G2 according to EN 10025:94

5.3 Microstructures of the analyzed steel are ferrite-pearlite with the maximum hardness of 182 HV10 at non-alloyed steel with the thickness 20 mm and maximum 198 HV10 at the steel with the thickness of 40 mm.

5.4 The mechanical characteristics of strength and deformability presents high values Rp0,2 of max 422 N/mm2, Rm = 540 N/mm2, A5 = max. 35% and Zmax=72% values which are specific to C-Mn steel alloy with high mechanical strength according to EN 10025:94

5.5 Current toughness of the steels of 20 and 40 mm thickness have the breaking energy values between 63 and 105 J more than the value of 27 J required by the standard EN 10025:94

5.6 Characterization of aging phenomena show that the steel with the thickness of 20 and 40 mm from the construction of hydraulic component have elevated level of the aging GI at the test temperature of +20°C (GI of between 22.2% and 55,5%) and at the test temperature of -20°C (GI between 78.1% and 90.8%), values which indicates that the aging phenomena are generally enhanced at the temperature of -20°C where all the degree values of aging are more than 50%, and the brittle-type fracture risk in this case is very high.

5.7 After welding the cut areas will be made 100% control on the welds lengths and in the absence of defects will starting to prepare technical documents for qualification of welding technologies under the current rules.

References [1]. Pascu, D.R şi alţii: Atlas metalografic pentru îmbinări

sudate, Editura Eurostampa, 2002, Timişoara România [2]. Trusculescu, M., Demian M.: Materials Handbook

Vol1. Structural Metallurgy, Editura Politehnica, 2006, Timişoara

[3]. Safta I., V: Încercările tehnologice şi de rezistenţă ale îmbinărilor sudate sau lipite, Editura Sudura, 2006, Timişoara, România

[4]. Pascu, D. R., Roşu, R., Deac, V., Chemical, structural, mechanical and corrosion characterization of the extraction pipes of natural gas, Welding and Materials Testing, No. 4, pp. 21-25, ISSN 2066-6586, 2011, Timişoara, România

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Structural and mechanical characterization of S355J2 weldable