Mechanical properties of shaft surfacing with micro-jet cooling

Welding surfacing process is very often used to apply a hardness or wear-resistant layer of base metal. It is very important method of extending the life of machines, tools, and construction equipment. Welding surfacing process is used to apply hardness and wear resistance. Main goal of that paper was describing possibilities of surfacing welding process with micro-jet cooling. Metallographic structure was analyzed in terms of micro-jet parameters. For getting various amount of martensite in this welding method it is necessary to determine the main parameters of the process such as: number of jets, i.e.:  diameter of stream of the micro-jet injector;  type of micro-jet gases;  micro-jet gas pressure. Welding with micro-jet technology was carefully tested for low alloy welding [1-13]. In the steel weld structure the best mechanical properties of weld correspond with chemical composition. Low amount of C (below 0.07%), Mn (below 0.7%), Si (below 0.4%), P (below 0.013%), S (below 0.014%), in WMD (weld metal deposit) correspond with good mechanical properties of welds, because of beneficial structure. Especially high amount of acicular ferrite (AF) in WMD is the structure that guaranties good impact toughness of welds. Having the most optimal chemical composition in weld it is only possible to get maximal 55% of AF in weld, but no more. It is why it was necessary to develop completely new welding technology to maximize of acicular ferrite (AF) content. Micro-jet technology gives chance to obtain artificially high amount of AF in low alloy weld (even on the level of 75%) [18-22]. Micro-jet technology also gives chance to obtain much higher amount of martensite after shaft surfacing [23-23]. The micro-jet technology was tested for steel welding with various micro-jet cooling conditions. A proper shaft regeneration has main influence on shaft exploitation [24-29]. Goal of that paper was to describe possibilities of innovative technology: shaft surfacing process together with micro-jet cooling.


Introduction
Welding surfacing process is very often used to apply a hardness or wear-resistant layer of base metal.It is very important method of extending the life of machines, tools, and construction equipment.Welding surfacing process is used to apply hardness and wear resistance.Main goal of that paper was describing possibilities of surfacing welding process with micro-jet cooling.Metallographic structure was analyzed in terms of micro-jet parameters.For getting various amount of martensite in this welding method it is necessary to determine the main parameters of the process such as: number of jets, i.e.:  diameter of stream of the micro-jet injector;  type of micro-jet gases;  micro-jet gas pressure.
In the steel weld structure the best mechanical properties of weld correspond with chemical composition.Low amount of C (below 0.07%), Mn (below 0.7%), Si (below 0.4%), P (below 0.013%), S (below 0.014%), in WMD (weld metal deposit) correspond with good mechanical properties of welds, because of beneficial structure.Especially high amount of acicular ferrite (AF) in WMD is the structure that guaranties good impact toughness of welds.Having the most optimal chemical composition in weld it is only possible to get maximal 55% of AF in weld, but no more.It is why it was necessary to develop completely new welding technology to maximize of acicular ferrite (AF) content.Micro-jet technology gives chance to obtain artificially high amount of AF in low alloy weld (even on the level of 75%) [18][19][20][21][22]. Micro-jet technology also gives chance to obtain much higher amount of martensite after shaft surfacing .The micro-jet technology was tested for steel welding with various micro-jet cooling conditions.A proper shaft regeneration has main influence on shaft exploitation [24][25][26][27][28][29].Goal of that paper was to describe possibilities of innovative technology: shaft surfacing process together with micro-jet cooling.

Experimental procedure
To obtain various amount of martensite in surface weld it was installed cooling micro-jet injector (only with one steam diameter of 40 µm and 50 µm) after welding head (Fig. 1).To analyze the surfacing with micro-jet cooling, there were chosen shafts of structural steel for carburizing 18H2N2 (18CrNi8) with a diameter of 28 mm.
Montage of shaft, welding head and micro-jet in-jector illustrates Fig. 1.
Fig. 1 Position for superficial welding with micro-jet cooling: 1welding head; 2micro-jet cooling injector, 3weld overlay machine shaft, 4handle, 5micro-jet gas pipe [21] Surface weld was prepared by welding with micro-jet cooling with varied parameters.Various micro-jet cooling gases (nitrogen, argon, helium) were tested in cooling process just after surface welding.There was not changing another important micro-jet parameter such as gas pressure (always on the level of 0.4 MPa).The main data about parameters of welding were shown in Table 1.Micro-jet gas pressure always 0.4 MPa 7.
Micro-jet diameter 40 µm and 50 µm Argon, nitrogen and helium were chosen for micro-jet cooling (with various diameter of 40 µm and 50 µm of micro-stream).MIG surface welding (argon) technology (with micro-jet cooling) was only used in this article.Argon is chosen as a shielded gas because of not oxidizing potential.Surface weld was prepared by welding with micro-jet cooling with varied parameters.After welding process surface was precisely grinded (Fig. 2).

Results and discussion
There were tested and compared various surface welds of standard MIG welding with new technology: MIG welding with micro-jet cooling.A goal of the study was to examine the varying structure of the typical surfacing shaft after welding.All tested welding processes were realized with varied micro-jet gases: argon, helium, nitrogen.A typical weld metal deposit had similar chemical composition in all tested cases.Micro-jet gas could have only influence on more or less intensively cooling conditions, but does not have greater influence on chemical WMD composition (except nitrogen amount in MWD), Table 2.For standard MIG welding and welding with two main micro-jet gases: argon and helium amount of nitrogen was always on the level of 50 ppm.For welding with nitrogen as a third micro-jet gas amount of nitrogen was higher, on the level of 70 ppm.After chemical analyses the metallographic structure was given.Example of this structure was shown in Table 3.Heat transfer coefficient of tested micro-jet gases influences on cooling conditions of welds (table 3).This is due to the similar conductivity coefficients (λ•105), which for Ar and N 2 , in the 273 K are not very various, respectively: 16.26 and 23.74 J/cm•s•K.Cooling conditions are rather similar when nitrogen and argon are chosen as a micro-jet gas.Helium gives much stronger cooling conditions due to the higher conductivity coefficients (λ•105), which for He is 143.4 J/cm•s•K.Micro-jet cooling does not have greater influence on chemical composition of weld.Only in case with helium micro-jet cooling there were observed traces of nitrides.There was firstly carried out metallographic structures for MIG surfacing with micro-jet cooling in terms on micro-jet parameters (Figs.3-6).It is not so easy to count precisely martensite amount such as other typical weld phases: acicular ferrite, grain boundary ferrite, side plate ferrite for low alloy welding [14][15][16][17][18][19][20][21][22][23][24][25][26][27].Martensite amount was only estimated.Nevertheless it is possible to present, that micro-jet technology is absolutely capable of structure steering (Table 3).Martensite amount is similar when nitrogen and argon are taken as micro-jet gases (Figs. 4, 5).Martensite amount is the highest when helium is taken as micro-jet gas (Fig. 7).After microscope observation a micro hardness was carried out (Figs. [7][8][9][10].Standard surface welding could not guaranty high hardness (Figs. 8).Fig. 7 Hardness of standard weld without micro-jet cooling Surface weld hardness of the shaft was decreased in terms of the distance from weld face, the maximum value was much below 500 HV.Much higher hardness values were observed after welding with micro-jet cooling (Fig. 8).Fig. 8 Hardness of weld after micro-jet cooling, argon used as micro-jet gas Shaft surface welding with micro-jet cooling (by argon) with one jet allowed to excide hardness under 500 HV.A little higher surface weld hardness values were observed after welding with nitrogen as a micro-jet gas (Fig. 9).Fig. 9 Hardness of weld after with micro-jet cooling, nitrogen used as micro-jet gas Surface shaft welding with micro-jet cooling (by helium) allowed to excide hardness under 550 HV, (Fig. 10).Fig. 10 Hardness of weld after micro-jet cooling, helium used as a micro-jet gas The micro-hardness inside the martensitic regions after welding with micro-jet cooling are higher than in standard weld, i.e. between 580 HV than 475 HV compared to 470 HV.The tribological properties of steel after thermo treating and micro-jet welding processes were obtained on the new elaborated research T-11 band [28].In the case studies, the T-11 tester was used to determine average: friction coefficient μ, and frictional force Tav.Each sample was tested five times The tribological tests show that micro-jets cooling affects the properties of the welds.Table 4 summarizes the results of tribological tests.
Based on the results stated in Table 4, it was found that the highest resistance to abrasive wear has the sample taken after welding with micro-jet cooling.Favourable micro-jet gases are nitrogen or helium.

Conclusion
The micro-jet surfacing technology was tested for surface welding with various micro-jet gases (nitrogen, argon, helium) and another micro-jet parameter (micro-jet gas pressure).Micro-jet technology could be treated as a On the basis of investigation it is possible to deduce that:  micro-jet-cooling could be treated as important element of MIG welding process;  it is possible to steer the metallographic structure (martensite, nitrides);  it is possible to steer the weld harness by various micro-jet parameters;  there is not great difference between the influence of argon and nitrogen on cooling conditions;  helium used for micro-jet cooling (instead of argon and nitrogen) is responsible of the highest hardness in all tested;  there were observed traces of nitrides when nitrogen was used for micro-jet cooling (instead of argon when nitrides were not observed);  the highest resistance to abrasive wear has the sample taken after welding with micro-jet cooling.

Fig. 6
Fig. 6 Martensite (aprox.80%) in weld after welding with micro-jet cooling (helium as micro-jet gas), magn.500x Fig. 11 A plot of the coefficient of friction between the time at which the strength was increased for sample made from deposit without micro-jet cooling

Table 2
Chemical composition of metal weld deposit

Table 3
Martensite in weld (argon, helium and nitrogen used as a micro-jet gas)