Ultrasonic influence on mechanical characteristics and metallography for naval welded steel.

Dumitrache, Constantin ; Barhalescu, Mihaela ; Oanta, Emil 等

1. INTRODUCTION

Submerged arc welded joints, like other welded joints, may contain defects which most of the time are cracks. In the narrow welded area, cracks may develop from non-homogeneities during service. Both, the joint performance and the life time of the structure depend on the size and location of the cracks, together with characteristics of the local microstructures and the level of mechanical strength mismatch between the weld region and the base metal, under certain service conditions.

The welding has heat-induced effects consisting of structural changes and plastic deformation. All these results are linked to the non-uniformity of the temperatures in the area between the weld and the base metal.

The characteristics of structural changes in HAZ depend on the following parameters: heat temperatures, chemical composition and cooling rate.

These changes may produce anisotropy of properties such as softening of HAZ if the steel is high alloyed, or the appearance of Wiedmannstatten structures if the steel is low alloyed.

Ultrasonic welding in condition of submerged arc improves mechanical resistance characteristics because it is supposed that ultrasonic energy transmitted by an electrode wire into metal liquid bath at resonance, breaks primary crystals forming into the weld and reduces grains' size (Susan et al., 2008).

In order to compare aspects regarding the mechanical characteristics and metallography of the NW, UW methods, the following stages were respected:

1) Welding--it was performed under both normal conditions (classical welding-NW) and ultrasonic activation of electrode wire (UW) with amplitude in welding zone A=10 x [10.sup.-6] m (Dumitrache et al., 2002);

2) Standard transverse tensile and bending-impact--the specimens were used to evaluate mechanical properties for each welded plate (***, 2002);

3) A microhardness survey--it was conducted across the welds beads, being used a Vickers microhardness testing machine (PMT-3 type) (Mitelea & Budau, 1992);

4) Concluding remarks--metallography specimens relieve changes of HAZ microstructures.

2. EXPERIMENTAL PROGRAMS

In this study, the base metal is a Romanian naval steel (low-alloy steel, type "A"), 8 mm thickness. Table 1 presents the chemical composition of the base metal and table 2 presents the composition of the welding wire (S12Mn2), the fusing agent (FB 10) and of the couple wire-fusing agent.

Mechanical tests (fig. 1, 2) were conducted at room temperature, each mechanical characteristics being in fact an average of three experimental values (Dumitrache, 2000).

Microhardness examination illustrated in figure 4 was done on the three different layers: layer 1 is the top of the weld bead, layer 2 is in the middle and layer 3 is located to the root of weld bead (Dumitrache, 2000).

The microhardness results were determinated with:

[HV.sub.01] = 0,1891 x F/[d.sup.2] (1)

where "F" is the compressive load (F=100 grams=0,98 N) and "d" is the distance measured on the indentation hardness, which was calculated with d=K x N, where K=0,309 is the constant of testing machine, and N is the number of divisions which was read on the scale division of testing machine (***, 2006).

The weld bead exhibits a microstructures comprising grain boundary ferrite with bainite and possibly martensite. Near the weld bead is present a coarse Wiedmannstatten structure (figure 3) which may contain defects (Salagean & Dragulescu, 1986).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

3. CONCLUSIONS

After UW the weld bead exhibits a fine microstructure of grain boundary ferrite and near the weld bead a fine Wiedmannstatten structure (fig. 3), which improve mechanical properties specially the plasticity mechanical properties (reduction in area and bending impact energy). To conclude, the mechanical strength properties (yield and tensile strength) were reduced, respecting the admissible range of values (Dumitrache, 2000).

4. ACKNOWLEDGEMENT

Part of the results presented in the paper use some of the accomplishments of the "Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective" ID1223 project, the supervisor being the National University Research Council (CNCSIS), Romania, (Oanta et al., 2007).

The future investigations are centered to explain which is the limit of ultrasonic energy (frequency) for optimization structural changes and mechanical characteristics, and improving an ultrasonic stress-relief method at welded joints.

5. REFERENCES

Dumitrache, C. (2000). Researches about ultrasonic influences on the metal welded structures, PhD Thesis, 182 pages, Field of science: Mechanical Engineering, 2000, 'Gheorge Asachi' University of Iasi, Romania

Dumitrache, C.; Comandar, C.; Susan, M. & Sabau, A. (2002). The influence of ultrasonic energy on the mechanical properties at the welded naval steel, Proceeding of EE&AE'2002 International Scientific Conference, pp. 127-130, ISSN 1311-9974, Rousse, 4-6 April, Bulgaria

Mitelea, I. & Budau, V. (1992). Materials and Heat Treatments for Welded Structures, pg. 408, Ed. de Vest, Timisoara, Romania

Oanta, E.; Panait, C.; Nicolescu, B.; Dinu, S.; Hnatiuc, M.; Pescaru, A.; Nita, A. & Gavrila, G. (2007-2010). Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective, ID1223 Scientific Research Project, under the supervision of the National University Research Council (CNCSIS), Romania

Salagean, T. & Dragulescu, D. (1986). Maximum microhardness from heat affected zone, Rev. METALURGIA, no. 12, 1986, ISSN 0461-9579

Susan M.; Bujoreanu G.; Dumitrache C.; Hanganu C. & Baciu C. (2008). A kinematical study of ultrasonic welding based on a system of stationary waves, Journal of Optoelectronics and Advanced Materials, Vol. 10, No. 6, June, p. 1425-1430, ISSN 1454-4164, Impact factor: 0,827

*** (2002). Metallic materials--Tensile testing--Part 1: Method of test at ambient temperature, SR EN 100021:2002, Standards Romanian Association;

*** (2006). Metallic materials. Vickers hardness test. Part 1: Test method, SR EN ISO 6507-1:2006, Standards Romanian Association

Tab. 1. Chemical composition of naval steel

Romanian naval steel, type "A"

Chemical C 0,117
composition [%] Si 0,041
 Mn 0,550
 P 0,002
 S 0,017
 Cr 0,015
 Mo 0,013
 Ni 0,012
 Al 0,016
 Cu 0,015
 W 0,056
 Fe 99,137

Tab. 2. Chemical composition

Chemical composition [%]

 Welding Wire, S12Mn2, C=0,11
 Romanian type Mn=2,00
 Si=0,09
 P=0,018
 S=0,027
 Cr=0,08
 Ni=0,26
 Cu=0,26

Fusing Agent, FB-10, Romanian Si[O.sub.2]=38
 type MnO=10,06
 [CaF.sub.2]=4,02
 CaO=20,01
 MgO=8,48
 [Al.sub.2][O.sub.3]=16,31
 Ti[O.sub.2]=1,50
 FeO=1,15
 P=0,066
 S=0,036
 Humidity=0,030

 Couple Welding Wire-Fusing C=0,12
 Agent Mn=1,443
 Si=0,353
 P=0,030
 S=0,019
 Cr=0,043
 Ni=0,057
 Cu=0,147
 Mo=0,019
 W=0,052

Fig. 1 Ultimate elongation and area reduction (experimental
data)

 Elongation, A5[%] Reduction in area, Z [%]

1-NW 7.5 43.5
2-UW 7 45

Note: Table made from bar graph.

Fig. 2. Bending impact energy; zone 1--weld bead centre; zone
2--HAZ; zone 3--two millimeters away from weld bead
(experimental data)

 zone 1 zone 2 zone 3

1-NW 24.3 20.3 21.3
2-UW 29.6 22.3 18.3

Note: Table made from bar graph.