The accumulated experience of the welded item maintenance shows that in the course of time the failure of their elements takes place. As a rule it occurs in the weld joints due to the influence of the temperature and power loads, different types of corrosion and other factors. It is conditioned on the fact that the weld joints are characterized by the structural heterogeneity and concentration of the residual stresses which do not comply with the connected parts of the steel structures.
The obtained structure and properties of the weld joints depend on the diffusion displacement of the alloying elements, which can determine the degree of the weld joint heterogeneity and phase-structural transformation process. It is important, that the boundary diffusion processes are very important for the weld joints. These processes are more active in the weld joints than in the body of the grain. In its turn it is related to the greater closeness of the crystalline structure imperfections on the grain boundaries. As a result, the concentration of the different types of the admixtures on the grain boundaries determines the possibility of the considerable change of metal properties of the welded areas, their behavior during the different types of treatment/ It is determined to a greater extend by the alloying element diffusion speed depending on the character and degree of steel alloying [1].
Thus one of the major tasks is the improvement of the weld joints quality as well as their reliability in view of the sharp necessity to reduce the emergency situations resulted in the high material costs aimed at their elimination. It is very important to obtain the full strength weld joint as well as the weld seam with the controlled properties, in particular due to the reduction of its mode of deformation and obtaining the properties, which are close to the properties of the parent metal to the maximum. It is possible to solve this task with the help of the deformation and thermal influence on the weld joint [2 - 4].
The purpose of this work is a study of the combined treatments influence, such as hot rolling deformation, low- temperature plasma treatment, on the quality of the weld joint.
For the examination the samples were made of steel grades 20, 20X, 18XGT and 30XGCA. The samples of steel grades 20X, 18XGT and 30XGCA were welded by the electrode of SB08G2C type (% : 0,076 C; 1,96 Mn; 0,88 Si; 0,014 S; 0,024 P; 0,044 Cr; 0,017 Ni; 0,022 Cu; 0,005 Al;0,005 As; 0,007 N), and samples of steel grade 20 were welded by the electrode of UOHI-13/45 type (% : 0, 11 C; 0,45-0,8 Mn; 0,2-0,3 Si; 0,03 S; 0,035 P).
The measuring of hardness showed that its distribution on the weld joint section is uneven (fig.1).It is conditioned by the heterogeneity of the formed structures (fig. 2).
Fig. 1. Distribution of hardness along the weld joints
Fig. 2. Microstructure of the weld seam and heat-affected zone of steel grades 20 (1), 20X (2), 18XGT (3), distance from the center of the seam, mm: a) 3; b) 9; c) 12; d) 15; x400. Number of grain according to GOST 5639 – 82 [1]
The above data show that with the increase of steel alloying degree the weld joint structure becomes more even, a grain in the heat- affected zones becomes somewhat finer. It becomes apparent in the hardness increase and thus in the beneficial effect on the further strengthening of the weld joints. A weld joint structure of steel grade 18XGT is the finest, it is especially reviled in the heat-affected zone and can be explained by the large alloying and titanium content in its composition which restrains the grain growth at high temperatures. The introduction of the additional alloying elements influences on the properties of the weld joints that is evident from the hardness increase along the weld joint with the increase of alloying.
The further weld joint treatment was carried out by the method of hot rolling deformation. The welded flats were preheated in the pipe electric furnace of T- 40/ 600 type, than the samples were kept in the furnace and deformed a weld seam by rolling up to the sample thickness level of parent metal at the laboratory duomill with the adjustable pressure screw with the rolling speed of 0,263 m/s and different pauses between the passes. The hot rolling parameters of the welded samples are shown in the table.
Table – Hot rolling parameters of welded samples
| Steel grade | Sample thickness, mm | Preheating temp. 0Ñ | Soaking time in furnace, min. | Period btw passes, s | Number of bloom passes |
| 20 | 9,5 | 950 | 19 | 12 | 1 |
| 20 | 9,5 | 950 | 19 | 31 | 2 |
| 20Õ | 10 | 950 | 15 | 11 | 1 |
| 20Õ | 10 | 950 | 15 | 19 | 2 |
| 18ÕÃÒ | 9 | 950 | 18 | 7 | 1 |
| 18ÕÃÒ | 9 | 950 | 18 | 42 | 2 |
| 30ÕÃÑÀ | 8 | 950 | 13 | 4 | 1 |
| 30ÕÃÑÀ | 8 | 950 | 13 | 15 | 2 |
Rolling of the welded seam to the level of the parent metal was conducted in the different ways: in the first case the rolling was conducted in one direction for 1 passage-way, in the second - rolling was conducted in the different directions in the “ reverse” mode for 2 passage-ways, than the samples were air cooled. After welding the Rockwell hardness along the weld joint was tested with the help of the TK-2 device according to GOST 6996-66 (Fig 3).
Fig.3 Distribution of hardness along weld joints: a) rolling in one direction; b) rolling in two directions (dotted line means welded seam)
It is obvious from the Figure 3 that the samples of steel grade 20 being welded by the more carbonaceous electrode than the samples of steel grade 20X, 18XGT and 30XGCA. They have more even characteristics along the weld joints after hot rolling in different directions. It is connected with the approximation of the chemical composition of the seam metal to the parent metal, whereas the samples of the rest steel grades have nonuniform properties after treatment, because the seam metal considerably differs from the welded metal. However the properties of the weld joints of steel grade 20X, 18XGT and 30XGCA are more uniform while rolling in the “ reverse” mode, than while rolling in one direction. It is related to the structural changes in the process of hot rolling depending on the number of passes and rolling direction.
The application of another technological process for hot rolling deformation treatment of weld joints of steel grade 20 showed another results. The welded flats were preheated to the temperatures of 950- 970C with subsequent soaking of samples in the furnace during 15 minutes and rolling deformation of the welded seam to the level of the parent metal with the pauses between passes of 6- 8 secs. Rolling of the welded seam to the level of the parent metal was conducted in the different ways: in the first case the rolling was conducted in one direction for 4 passage-ways, in the second – the rolling was in the various directions in the “ reverse” mode for 4 passage-ways with subsequent air cooling of the samples.
Hot rolling deformation resulted in the different structure and properties of the welded bars. They differed from the initial ones. In particular, the samples before hot rolling deformation had consertal and heterogeneous section structure. The microstructure of weld joint zone, consisting of ferrite and perlite , showed extremely heterogeneous structure, especially in the normalization zone, where the grain score was within the range of #6-4 according to GOST and grains had heterogeneous structure, which was not typical for normalization zone. After hot rolling deformation in one direction the weld joint structure became more heterogeneous and nonequilibrium. It was also the evidence of the lower quality of a weld joint, however after deformation in the different directions the structure became more homogeneous, the processes of recrystallization in the structure of the weld joint took place. It resulted in the leveling of the properties of the sample section, improvement of its quality and strength and thus in the partial strengthening of the weld joint [5] .
The further goal of the Master's Thesis is the research of the influence of the low temperature plasma treatment and surface impregnation on the structure and weld joint properties.