Abstract
Based on Q345 steel, high-strength low-alloy surfacing layers with Nb content ranging from 0.0041 to 0.26 wt-% were prepared by adding Nb element into electrode coating and manual electric arc welding, the microstructure as well as mechanical properties of surfacing layers were evaluated and analysed. The experimental results show that with the increasing of Nb content, the grain size becomes smaller meanwhile the microstructure distribution is more uniform, the size of hard phase martensite/austenite gradually decreases and the NbC precipitates increases. The yield and tensile strength greatly increase for the fine-grain strengthening of Nb and the precipitation strengthening of NbC, moreover, the impact toughness is significantly improved due to the microstructure variation and obvious grain refinement.
Introduction
Surfacing is one kind of welding method which can obtain the deposited metal with special properties on the welding surface. 1 Many traditional welding methods, for instance, manual electric arc welding, submerged arc welding and gas shielded arc welding, have been used in surfacing. 2 Among all the methods, manual electric arc welding mainly depends on the electrode coating to transfer the required alloying elements into the surfacing metal. The experimental studies show that the interaction between the alloying elements and iron or carbon, also between alloying elements, can change the internal structure of alloy and then improve the mechanical properties of surfacing metal.3,4 Micro-alloying elements, such as Nb, V, Ti, can be formed as fine carbides and nitrides or carbonitrides in the steel, the particles of precipitates could prevent the growth of austenite grain by means of pinning at grain boundaries in reheating process. 5 It is well known that the micro-alloying element Nb is added to a wide range of steels for improving microstructure and properties by precipitation strengthening and grain refinement,6–8 and it has the strongest fine-grain strengthening effect of all micro-alloy elements. Besides, the Nb element can further improve the strength of steels via phase transformation control. In addition, the properties, shape, size and distribution of the precipitates and their co-lattice degrees with the matrix phase determine the precipitation hardening contribution to the material strength. 9
The Nb-bearing steels have shown up many advantages, such as the balance of high strength and excellent toughness, and less cost, 10 in particular, Chen et al. 11 have proved by experiments that the high-Nb X80 pipeline steel has excellent welding properties. Therefore, many scholars have carried out researches on the application of Nb element, including the influence on microstructure and properties of the Nb-bearing surfacing layers. Recently, Nb element was successfully added into nickel-based laser hardfacings to increase fracture toughness.12,13 Furthermore, the study of high speed steel laser hardfacings shows that Nb has a high impact on the hardfacing microstructure and its mechanical properties. 14 Manual electric arc welding is one kind of mature and traditional surfacing methods, however, there are very few experiments using the low-alloy electrodes. Thus, considering the impact of Nb element on high-strength low-alloy (HSLA) steels, adding different content of Nb element into low-alloy electrode coating, the HSLA surfacing layers based on Q345 steel with different Nb content were prepared using the methods of manual electric arc welding and chemical composition transition of electrode coating, in order to investigate the effect of Nb element on microstructure and mechanical properties of HSLA surfacing layers.
Materials and experimental procedures
Materials
Chemical composition of Q345 steel (wt-%)
The ferroniobium content of CHE607GX electrodes
Chemical compositions of surfacing layers (wt-%)
Experimental procedures
All the samples were cut by wire cutting machine, the metallographic specimens were prepared perpendicular to the length direction of surfacing test specimens, whereas the tensile and impact specimens were machined parallel to the length direction. The metallographic specimens of each surfacing layer were mechanically polished and etched with a 3% nital solution, for microstructure observation by DM6000M LEICA optical microscopy and JSM-7001F field emission scanning electron microscopy (SEM), and the average diameter of grains were measured by the metallographic analysis software. In addition, the surfacing layers microstructure was further determined by means of transmission electron microscopy (TEM). Thin foils were prepared by cutting 0.5 mm slice and mechanically thinned them to 50 μm thickness range by metallographic sandpapers, then 3 mm discs were punched from the foils and electro-polished using an electrolyte solution of 7% high acid anhydrous in acetic acid at the double electrolytic voltage of 28–30 V and the current of 72–80 mA. The discs were examined by the JEOL 2010 TEM, furthermore, the precipitation location and size of Nb-bearing precipitates were analysed. The rod tensile specimens were machined with a gage diameter of 10 mm and gage length of 50 mm, and the tensile tests were conducted by SHT4605 universal testing machine at the room temperature of approximately 20°C, and all these tests were done at a crosshead speed of 2 mm min−1. The specimens for impact test were machined to standard Charpy V-notch impact specimens being in the dimension of 10 mm × 10 mm × 55 mm, the impact toughness was measured by JBNS-800 impact testing machine at −20 and −40°C separately after the specimens were cooled to the specified temperature in the CDW model impact test low temperature instrument, then the impact fracture micromorphology was observed with ZEISS EVO 18 SEM. The micro-Vickers hardness of surfacing layers was measured by utilising an HVS-1000 vivtorinox hardness-testing machine employing a load of 1000 kgf, and the indentation area covered the typical microstructure. The toughness and hardness values presented here are an average of three and six measurements, respectively.
Results and discussion
Effect of Nb content on surfacing layers microstructure
The microstructure of each surfacing layer with different Nb content is shown in Fig. 1, and the SEM microstructure is shown in Fig. 2. Combined with Fig. 1 and Fig. 2, it can be seen that the microstructure of surfacing layers mainly consists of ferrite, bainite and martensite/austenite (M/A) islands, the M/A constituents with the shape of granular mainly distribute at the grain boundaries, and the average size of grains were 16.65, 15.73, 12.81, 11.75, 10.15 and 9.03 μm as the Nb content is 0.0041–0.26% through the metallographic analysis software. Hence, with the increasing of Nb content of the surfacing layers, the quasi-polygonal ferrite (QPF) in microstructure increases gradually, the content of lath bainite (LB) and the grain size decrease, meanwhile the microstructure distribution is more uniform. When the Nb content are 0.0041 and 0.046%, due to the precipitation of proeutectoid ferrite along the prior austenite grain boundaries, the QPF as reticulate with uneven distribution is less, and it can be obviously observed in the prior austenite grain boundaries, moreover, the size of QPF is varying, at this time the microstructure is mainly composed of QPF, LB and M/A islands, as shown in Fig. 1a and b and Fig. 2a and b. It is mixed type microstructure of QPF, LB, granular bainite (GB) and M/A constituents with the Nb content of 0.11%, the fraction of LB significantly reduces while the QPF distributes uniformly. The grain size of surfacing layer becomes more uniform and refines significantly compared to the Nb content of 0.0041 and 0.046%, as shown in Fig. 1c. The LB disappears while the Nb content varies from 0.17 to 0.26%, the microstructure is consist of QPF, a small amount of GB and M/A islands, besides, the grains become fine gradually along with the increase of Nb content, as shown in Fig. 1d–f and Fig. 2d–f. It reveals that with the increase of Nb content, the grain size gradually decreases because more NbC precipitates pinning grain boundaries can effectively refine austenite grains, and the fine-grain strengthening effect of Nb element is strongest in micro-alloying elements, thus, its grain-refinement effect is very significant. Meanwhile, these precipitates can further provide more ferrite core position due to the refinement of austenite grain. In addition, it has been reported that the Nb addition in low-carbon steel can obviously hinder the bainite sheaves transformation, however promote the formation of low temperature transformation products, such as GB,8,15,16 and small austenite grains can retard bainite transformation, particularly make more restrictions to grow bainite sheaves.8,17,18 Therefore, as mentioned previously, the ferrite content in the surfacing layers grows while the LB decreases gradually with the increase of Nb content, and there is no LB but a small amount of GB above 0.17% Nb.
Optical microstructure of surfacing layers with different Nb content. a ω(Nb) = 0.0041%; b ω(Nb) = 0.046%; c ω(Nb) = 0.11%; d ω(Nb) = 0.17%; e ω(Nb) = 0.19%; f ω(Nb) = 0.26% SEM microstructure of surfacing layers with different Nb content. a ω(Nb) = 0.0041%; b ω(Nb) = 0.046%; c ω(Nb) = 0.11%; d ω(Nb) = 0.17%; e ω(Nb) = 0.19%; f ω(Nb) = 0.26%
Figure 3 shows the TEM microstructure of the surfacing layer specimens. It can be seen that when the Nb content is 0.0041%, the LB in surfacing layer is the width of about 100–200 nm, the regular parallel strips have large aspect ratio and are arranged in bundles according to definite orientation, moreover, the boundaries of strips are clear and straight. The dislocations are evenly distributed in the slab, a large amount of mutually entangled dislocations can be observed at the grain boundaries by reason that these boundaries can hinder the dislocation slip. As hard phase, the M/A constituents located at the boundaries and internal of grains represent as the block or sharp corner, the size of M/A constituents at the grain boundaries is about 100–200 nm, whereas it is about 600 nm in the grains. However, the Nb-bearing precipitates are not observed for the very low Nb content, as shown in Fig. 3a–d. It can be seen from Fig. 3e that the mixed microstructure of surfacing layer is composed of QPF, LB, M/A islands and NbC precipitates with the Nb content of 0.11%, the LB and QPF distribute alternately, the M/A constituents founded at the boundaries between LB and QPF are approximately 100 nm, and the granular NbC precipitates from the grains. It can be observed in Fig. 3f that the dislocations intertwined with each other are distributed around the QPF grain boundaries. The LB is not found in the surfacing layer while the Nb content is 0.19%, the M/A constituents which are the size of 50 nm exist in the QPF grain boundaries. Furthermore, high density dislocations are arranged at the grain boundaries and stacked to form a dislocation wall, and some of the dislocations which slip forward as discrete dislocation or fold shape are distributed in the grains
19
; on account of Nb-bearing carbon-nitride being powerful obstacle, it is difficult for dislocations to cut off the precipitates whether or not the precipitates co-lattice matrix, therefore, the NbC precipitates pinning dislocations can be seen in QPF, as shown in Fig. 3g, h. The TEM morphology and the corresponding energy spectrum analysis of surfacing layers are shown in Fig. 4, and it is known that all the precipitates are NbC. The granular NbC precipitates in Fig. 4a with the size of about 15 nm separate out from the LB as a linear arrangement when the Nb content is 0.11%; the granular NbC precipitates which are the size of about 25 nm separate out from the QPF grain and boundaries as the Nb content is 0.19%, and a little bit larger than which are with the Nb content of 0.11%, as shown in Fig. 3g, h and Fig. 4b. Thus, it can be figured out that surfacing layer dislocations primarily accumulate at grain boundaries, besides, a part of NbC precipitates lead to the dislocations entanglement by occasion of pinning dislocations. The hard phase M/A constituents both exist at the grain boundaries and inside the grains, but mainly at boundaries. When the Nb content is as low as 0.0041%, the Nb-bearing precipitates are not found in the surfacing layer; along with the growing of Nb content, the size of NbC precipitates increases while the M/A constituents decreases gradually.
TEM microstructure observation of surfacing layers. a–d ω(Nb) = 0.0041%; e, f ω(Nb) = 0.11%; g, h ω(Nb) = 0.19% TEM observation and energy spectrum analysis of surfacing layers precipitates. a and c ω(Nb) = 0.11%; b and d ω(Nb) = 0.19%
Effect of Nb content on mechanical properties
Tensile testing results
Tensile testing data of each surfacing layer with different Nb content
Impact testing results and SEM observation of impact fracture surface
The impact absorbed energy at −20 and −40°C of each surfacing layer are shown in Fig. 6. It can be seen from Fig. 6 that the impact absorbed energy increases from 25 to 165 J at −20°C and increases from 15 to 140 J at −40°C while the Nb content of the surfacing layers increases from 0.0041 to 0.26%. The impact absorbed energy at −40°C is lower than which is at −20°C with the same Nb content because the materials toughness reduce with the decrease of temperature. When the Nb content are 0.0041 and 0.11%, the larger size and sharp M/A constituents can damage to the toughness; the impact absorbed energy increases rapidly as the Nb content is more than 0.17%, it is mainly derived from the disappearance of LB, the increase of QPF and the obvious refinement of grain size in surfacing layers. With the growing of Nb content, although the increasing of NbC precipitates size may damage the plasticity and toughness of surfacing layers, the strength increase meanwhile the impact toughness is improved, because the grain refinement and the diameter of NbC precipitates is not more than 30 nm. The impact fracture surface of the surfacing layers at −20 and −40°C are shown in Figs. 7 and 8, respectively. It can be seen that the impact fracture of surfacing layers at −20 and −40°C are quasi-cleavage fracture, which shows the mixed morphology of quasi cleavages, micropores and tear ridges, the tear ridges composed of micropores distribute between the quasi-cleavage planes, the existence of tear ridges illustrate that there is micro plastic deformation of surfacing layers in the fracture process. When the Nb content is from 0.0041 to 0.19%, the relatively short river pattern is on the planes of quasi-cleavage fracture, while the river pattern disappear as the Nb content is 0.26%. The number of tear ridges and micropores increase, as well as the impact toughness of surfacing layers is improved along with the growing of Nb content, which are in accordance with the change trend of the impact absorbed energy values.
The stress–strain curves of surfacing layers The impact absorbed energy of each surfacing layer at −20 and −40°C SEM morphology of impact fracture surface of each surfacing layer at −20°C. a ω(Nb) = 0.0041%; b ω(Nb) = 0.046%; c ω(Nb) = 0.11%; d ω(Nb) = 0.17%; e ω(Nb) = 0.19%; f ω(Nb) = 0.26%
Micro-Vickers hardness-testing results
Figure 9 shows the micro-Vickers hardness of surfacing layers with different Nb content, the hardness values distribute as ‘N’ shape curve. When the Nb content is from 0.0041 to 0.11%, the hardness values increase quickly, it is relatively high due to the existence of ‘hard phase’ LB in microstructure. Here, although the decreasing of ‘hard phase’ LB, the dispersion of NbC has great contribution to hardness, which leads to the surfacing layers hardness rapidly increasing. When the Nb content is from 0.11 to 0.17%, the hardness values decrease sharply from 232.8 to 207.3 HV, it reaches the maximum value at 0.11% Nb, with the mixed microstructure of QPF, LB, GB and M/A constituents, moreover, the QPF matrix uniformly distributes some LB, and the M/A constituents disperse in the grain boundaries. The good matching of ‘soft phase’ and ‘hard phase’, besides, the common function of hard M/A constituents and NbC precipitates, make the surfacing layer have the highest hardness value. However, the hardness value quickly reduces about 0.17% Nb because of the disappearance of LB. When the Nb content is higher than 0.17%, the hardness values increase very slowly, and for the different microstructure, it is relatively low compared with the hardness values below 0.17% Nb. Since the gradually increasing size of NbC precipitates with the growing of Nb content, the hardness values also exhibit a trend of slow increasing despite the microstructure of ‘soft phase’ QPF.
SEM morphology of impact fracture surface of each surfacing layer at −40°C. a ω(Nb) = 0.0041%; b ω(Nb) = 0.046%; c ω(Nb) = 0.11%; d ω(Nb) = 0.17%; e ω(Nb) = 0.19%; f ω(Nb) = 0.26% Micro-Vickers hardness of each surfacing layer with different Nb content
Conclusions
Based on the current investigation on the Nb-bearing HSLA surfacing layers, the following conclusions can be drawn:
With the increase of Nb content in surfacing layers, the grains become fine and the distribution of microstructure becomes more uniform, the LB decreases while the QPF increases gradually, the size of M/A constituents reduces however the NbC precipitates increases. When the Nb content is as low as 0.0041%, the Nb-bearing precipitates are not observed in the surfacing layer; when the Nb content is more than 0.11%, the GB is visible in microstructure, the grains significantly refine and become more uniform; when the Nb content is higher than 0.17%, the LB disappears. When the Nb content increases from 0.0041 to 0.26%, the yield strength of surfacing layers increases from 563 to 742 MPa and the tensile strength increases from 641 to 812 MPa. The impact absorbed energy values of surfacing layers increase from 25 to 165 J at −20°C and increase from 15 to 140 J at −40°C. The micro-hardness values increase rapidly with the Nb content from 0.0041 to 0.11%, and then decrease quickly, it increases slowly while the Nb content is from 0.17 to 0.26%. The strength and toughness of Nb-bearing HSLA surfacing layers are related to the grain refinement of Nb element and the precipitation strengthening of NbC precipitates, especially the effect of fine grain, which has great influence on the microstructure and mechanical properties of surfacing layers.