Raising quality of maintenance and control of metallic structures in large-load technological machines

Active growth of coal extraction and underinvestment of coal mining in Russia lead to the fact that technical state of more than 86% of technological machines at opencast coal mines is unacceptable. One of the most significant problems is unacceptable state of supporting metallic structures of excavators and mine dump trucks. The analysis has shown that defects in these metallic structures had been accumulated for a long time. Their removal by the existing method of repair welding was not effective – the flaws reappeared in 2–6 months of technological machines’ service. The authors detected the prime causes that did not allow to make a good repair welding joint. A new technology of repair welding had been tested and endorsed, and this allowed to reduce the number of welded joints’ flaws by 85% without additional raising welders’ qualification. As a result the number of flaws in metallic structures of the equipment had been reduced by 35 % as early as in the first year of using the new technology.


Introduction
Coal is one of the most significant resources that stabilize the situation in Russian power industry. The advantages of opencast coal mining are obvious because 70 % of the total amount of coal mined Russia-wide is extracted at opencast mines. Kuznetsk Basin (Kuzbass) is one of the largest in Russia and in the world [1] Within the last five years there is positive increase in exported coal amounts, and Kuzbass places Russia to the third place among the most significant coal exporters after Australia and Indonesia. Kuzbass coal is exported to 85 countries of the world [2]. At the same time within the last twenty-five years investment of coal industry were insignificant. Even in 2010 which was the most stable year they made about 5 % of coal sales revenue [3]. Although despite negative investment environment out-of-date technological equipment and technological devices were replaced. But this replacement was insignificant for excavator fleet and dump truck fleet. That is the reason why opencast coal mines still use life-expired technological machines. This lead to the fact that 70 % of equipment practically every year undergoes safety review performed by experts, and the technical state of more than 86 % of equipment is unacceptable. Considering the tendencies of early 2000s which were based on total replacement of excavators produced in Russia with hydraulic excavators of foreign origin it may be said that this strategy had failed to prove its value. The advantages demonstrated by the imported equipment come to nothing by the fourth or fifth year of service. Thus introduction of imported equipment required significant current operating costs besides capital expenditures, and tough interrepair maintenance requirements caused the necessity of the whole maintenance system reconsideration. Thus in most of the cases such equipment undergoes post-warranty maintenance and requires permanent presence of a technical-2 1234567890 ''"" Accumulation of flaws had been induced by the fact of their untimely removal, and thus within six months of the flaws described in Table 1 29% of flaws by number and 28% by extension was removed. The least number of flaws were removed at AO Chernigivets -13% by number and 18% by extension. The most numberat ZAO Prokopievskiy Ugolnyi Razrez -51% by number and 45% by extension.
More than 90 % of flaws detected in metallic structures were removed several times but reappeared. Along with this ultrasonic testing showed that in more than 98 % their was a lack of penetration in the welded joint. Figure 1 shows the photo of a typical repair welded joint from backside. As it can be seen a lack of penetration extends through the whole length of the joint.

Theory
Unlike welding new metallic structures where welders' task is prepared groove of a future welded joint, the process of repair welding is organized in the way that the welders should groove and prepare the metallic structure for welding themselves. The main factors that complicate the process and do not allow to groove the welded joint in accordance with the National State   All this, complicating the work of the welder who is not familiar with the groove, leads to a typical mistakenarrow groove with leaving some metal for fusion (Figure 4), and it does not allow complete root penetration with a temper bead.

Experimental results
In order to get precise information about the causes of reappearance of the flaws on repair welded joints and to detect the reasons of lack of root penetration a special methodology allowing to model the typical working conditions for the welder making repair welded joints had been developed. Multipass welding of metallic specimens with different thickness at their different locations was modelled. Welded joints' groove was performed by the welders themselves. Welders of the 5 th and 6 th categories worked with metallic specimens 16 mm thick and welders of the 3 rd and 4 th categories worked with specimens 6 mm thick. The criteria of the specimens' assessment met the instructions for capital repair of excavators ESh and EKG and dump trucks Belarusian Auto works 7555 и Belarusian Auto works 75131. The last additional criterion was evaluation of the welded joint for the presence of welding stress using the metal magnetic memory method. 193 welders of AO HK SDS-Ugol enterprises had participated in the experiment. The analysis of more than 80 m of welded joints in 338 welding specimens showed the results given below in Table 2. From the Table 2 it can be seen that the dependence of the number no-go specimens' number on the welders' qualification is insignificant. On the specimens 16 mm and 6 mm thick respectively were observed: irregular ripples (91 % and 67 %), deviations of the welded joint size form the required by the National State Standard (100 % and 99 %), lack of root penetration through the full length of the welding joint (41 % and 46 %). The chart presented on Figure 5 shows the results of the assessment of the welded joints quality. The welded joints were done by the workers of AO HK SDS-Ugol enterprises. The number of specimens of the required quality according to visual and dimensional test done only by the welders of OOO Shakhtoupravleniye Mayskoye was 5.6 %. All other enterprises showed practically zero result. The specimens of ZAO Prokopievskiy Ugolnyi Razrez welders had passed visual and dimensional test and made 38.5% of their total amount. Only 5.6 % of specimens done by the welders of OOO Shakhtoupravleniye Mayskoye met the joint requirements of V&DT and UT.
The results of analysis of flaws' distributions according to the welders' categories shown on Figure 6 show that the quality of the welded joints does not depend on the welders' qualification. Thus on the one hand welders of the 6 th category had expectedly more specimens meeting the norms of V&DT (11.2 % more than the welders of the 5 th category), on the other hand they had less (2.8 % less) welded joints meeting the norms of UT than welders of the 5 th category. And only 1.1% of specimens done by the welders of the 5 th category had met the joint norms of V&DT and UT.  The results of the experiment with welding specimens evaluation had therefore shown that there were no specimens meeting the norms set by documentary standards. The most occurring flaws were those connected with lack of root penetration (Figure 7, a) and form deviations of the welded joint surface (Figure 7, b).

Discussion
More than 91 % of flaws detected by V&DT are removed by deseaming of the weld face (if necessary with backing weld and subsequent deseaming). This method allows to remove such flaws as irregular ripples, beadings tough transition to the base metal, excessive height of the crests step structure of the welded joints and undercuts. It does nt require expensive raising of welders' qualification and increases solidity of welded joints due to removal of macro-and micro acceptable flaws where welding stresses are consentrated. In the conditions of alternating strains (where welded joints usually function) it leads to the loss of their operational life due to appearing and extension of fatigue cracks. Removal of the flaws detected by UT is no less significant problem. The analysis of welded specimens' ultrasonic testing allowed to conclude that 85 % of flaws are flaws of groove (preparation for welding). They are not directly caused by the qualification of the welder making a joint, but to a greater extent they are flaws of a specialist preparing for the welding works. The solution of this problem lies in the area of repair conducting technology. Thus introduction of the method of slotted groove with installing a background plate allows to solve the problem (Figure 8). For practical approval and introduction of this technology there was an order for the whole enterprise at ZAO Prokopievskiy Ugolnyi Razrez: during the repair works on metallic structures welding joints' groove with installing a background plate is obligatory. This allowed as early as in the first year of functioning to reduce the number of detectable flaws in metallic structures of the technological machines on average by 35 % in comparison with other enterprises of AO HK SDS-Ugol (Figure 9).