The Many Facets and Complexities of 316L and the Effect on Properties Ingrid Hauer Miller Höganäs AB, Höganäs, Sweden state and country Ingrid.hauer@hoganas.com, +46702066244 Abstract One of the most widely accepted and versatile materials used with many coating deposition methods is austenitic stainless steel 316L. This is due to the combination of good corrosion and mechanical properties that are suitable for numerous varying applications. As a result of our familiarity with 316L we assume the properties achieved will be within the expected frame as described in the literature. Deposition techniques and component size and shape have an impact on the heating and cooling variations which will then affect the microstructure of the final coating. This in turn will result in variations in the properties achieved. Some of these numerous factors such as: deposition technique, component geometry and size, impurities in the substrate and chemistry of the substrate will be considered. The influence of these factors on the microstructure and the definitive properties of 316L will be presented. Introduction Welding provides very high bond strength between deposit and substrate and can be applied in thickness greater than most other techniques; typically in the range 2-20 mm. Welding is often used for resurfacing. These processes involve application of heat to the processed component and depending on the material from which it is made and its conditions, certain precautions may need to be taken. During fusion weld surfacing, the coating material is raised to its melting point, which means that metals and alloys used for the purpose must have a melting point similar or less than the substrate materials. Exceptions of this general rule are found in friction surfacing and explosive cladding. Other coating materials with higher melting points, such as ceramics, may be applied by thermal spraying processes. The welding methods used in this investigation are PTA and laser cladding. Laser cladding utilizes a high power laser beam as a heat source for the melting and fusing of the substrate and filler materials. As the laser is a beam of electromagnetic radiation, all of the injected energy is confined within a small spot with no diffusion due to convection from hot gasses or electromagnetic forces taking place. As a result, the heat input to the part is minimised and higher welding quality and efficiency is attained. Filler metal is added in many ways with the most common being powder injection. The creation of a melt pool where a fine film of base material is melted and the filler material is injected, results in the formation of a welding bead which is strongly adhered to the substrate. The dilution in the deposit is around 5%. The high intensity of the laser beam and the associated low heat input results in minimised distortion, residual stresses and base metal degradation. Furthermore, the higher cooling rates attained result in finer microstructures and smaller heat affected zones leading to improved mechanical properties. These facts allow the processing of critical parts which could not be repaired in the past with other fuse welding techniques. This opens new opportunities for performance enhancement and cost reductions through the salvaging of parts which were previously scraped. Furthermore, the impeccable control of overlay thickness and low overlay roughness minimises the machining allowances and hence lowers the production or repair costs. The plasma transferred arc, (PTA) process uses an argon shielded tungsten arc as the source of energy. The powder is introduced into a combined arc/plasma stream to form a molten pool on the workpiece. Additionally, the arc between the workpiece and gun produces surface melting of base material and dilution of 5 to 15% in the deposit is typical. This method should not be confused with plasma spraying which used a non-transferred arc to generate the heating plasma for spraying powders. The main differences between the two methods are: - Higher heat input in PTA compared to laser, which will also result in a greater heat affected zone and higher dilution - Faster cooling rate for laser which will give a finer microstructure, resulting in higher hardness and superior wear resistance. When the same coating method is used, the final quality of the clad is depending of several factors, such as: material used for cladding, component geometry, component size, impurity size and number. The purpose of this work is to show how the different factors influence the final quality of the coating, when the stainless steel 316L is used.
The differences in the coating properties are also presented when two different methods are used: laser cladding and plasma arc welding (PTA). Experimental Method The coating material used in this investigation is the austenitic stainless steel 316L with the composition shown in Table 1. The surfacing was made using laser cladding and PTA methods on low alloyed carbon steel substrates. Table 1: 316L composition. C% Si% Fe% Cr% Ni% Mo% Mn% 316L <0.03 0.8 base 17 12 2.5 1.5 cooling cycles. Finally the samples were plane ground and polished with 9µm subsequently followed by 3µm and lastly 1µm DP (diamond polish) suspension. The etching agent used was glyceregia which consists of 45 ml glycerol 98% (purum), 15 ml HNO 3 and 30 ml HCl. The samples were evaluated metallographically by LOM and SEM. The hardness HV10 was measured and calculated as an average of 7 measurements. Results and Discussion The pictures of the three laser coated sample together with the microstructure can be seen in Fig. 1, 2 and 3. All tests were performed in the Höganäs laboratory. The laser cladding was performed on a Coherent direct diode laser, 4 kw, 808 nm wavelength. Substrate was 1018 steel in 3 forms and the clads were applied in a single layer. Shapes used are as follows: - 70 x 100 x 10 mm (w x l x t) plate - 50 x 200 x 20 mm (w x l x t) plate - 25 mm ID x 30 mm OD tube Same parameters were used for all the laser cladding tests: - Power 3kW - Cladding speed 5 mm/s - Feed rate 30 g/min - Feed gas (Ar) 3 l/min - Shield gas (Ar) 10 l/min The PTA used was Hettiger with HP302 torch. The PTA weld was applied on 1311 low carbon steel plate measuring 50 x 200 x 20 mm (w x l x t). The parameters used for the PTA tests were: - Current: 125 A on the first layer 115 A on the second layer - Welding speed 12 mm/s - Feed rate 30 g/min - Feed gas (Ar) 1 l/min - Shield gas (95%Ar + 10%H) 12 l/min - Plasma gas (Ar) 1,5 l/min - Weld width 10 mm Figure 1: 316 as laser coated on OD30/ID25 tube Dilution was measured on the top layer using an XRF Niton XL2 gun for the laser cladded coatings and EDS method for the PTA ones. Metallographic test samples were prepared by cutting the samples perpendicular to the welding direction. These were placed in a mounting press which was filled to 1/3 with a glass fibre resin followed by 2/3 with Bakelite resin. The samples were processed under high pressure during the heating and
The microstructure of the coatings consists of austenite and delta ferrite. Fully austenitic stainless steels are known to be prone to hot cracking during welding. In order to prevent the occurrence of hot cracking, 4-10% ferrite should be present in the finished weld. This can be achieved via laser cladding processes due to the high cooling rate [1]. Figure 4 displays an even dendritic structure with coarser dendrites, which have longer time to grow due to the slowest cooling rate. This component was too small and overheated during welding which lead to slower cooling rate compared to the other components. Figure 5 shows a mostly even structure, but finer due to faster cooling rate. In case of the largest mass component, Fig. 6, the structure obtained was the most uneven compared to the smaller ones. Figure 2: 316 as laser coated on 10mm thick plate Figure 4. Dendritic structure on tube Figure 5. Dendritic structure on the 10 mm thick plate Figure 3: 316 as laser coated on 20 mm thick plate Each of the coatings in the previous figures show an overview of the coatings microstructure.
Figure 6. Dendritic structure on the 20 mm thick plate More etched pictures of the different microstructures are presented in Fig. 7, 8, and 9. Figure 9. Microstructure of coating performed on 20 mm thick plate The microstructures of the coatings performed on the plates were very similar (see Fig. 8 and 9). When the coating was applied to the smallest component, the tube, a lower amount of delta ferrite was obtained as a result of the lower cooling rate. The outcome was a small hot crack in the structure which is illustrated in Fig. 7. The PTA and laser clad samples were compared on the 20 mm thick plate. The microstructures are shown in Fig. 10 Figure 7. Microstructure of coating performed on tube Figure 8. Microstructure of coating performed on 10 mm thick plate Figure 10. Microstructure of 316L as laser welded (top) and PTA welded (bottom)
Figure 10 displays an altered size of the dendritic structure: finer in the laser cladded samples due to the faster cooling rate. Material 316L when laser welded shows excellent pitting corrosion, which was presented in an earlier paper [2]. A finer structure indicates better corrosion resistance compared to a coarser one, [3] and [4]. This should be further investigated for more clarification in the future. The amount of delta ferrite in the PTA and laser cladded samples has been investigated and the etched structures are shown in Fig. 9.Murakami etching was used to reveal the delta ferrite. the weight percentage of austenite stabilizing elements. By entering the Ni-equivalent over the Cr-equivalent for stainless steel into a diagram according to WRC, one is able to find the content of austenite and ferrite in the resulting microstructure. The WRC diagram is today accepted as an improved version of the Schaeffer or the De-Long diagram. The composition of both coating has been calculated using the EDS method for all the element beside C and N. These two elements have been chemical analyses in the coating using the IR analyze method. Figure 12. Chemical composition of the laser welded (top) and PTA welded sample (bottom) After entering the range of the standard analysis and the actual analysis, the Ni-equivalent and the Cr-equivalent are calculated and shown in a diagram (see Fig. 13) Figure 11. Microstructure of 316L as laser welded (top) and PTA (bottom) showing the delta ferrite (etched in Murakami) Figure 11 demonstrates a similar amount of delta ferrite in both samples. The WRC-1992 diagram [5] provides information on the welding properties of the various types of microstructure as a function of what alloying elements they contain. Chromium equivalent is calculated using the weight percentage of ferrite stabilizing elements and Nickel equivalent is calculated from
Figure 14. Line analysis of the laser coating (top) and PTA coating (bottom) As it can be seen in Fig. 14, a steeper curve is obtained when the laser process is used. Figure 13. WRC-1992 diagram for the laser welded sample (top) and PTA welded sample (bottom) showing the percentage of delta ferrite. The WRC diagram shows the same amount of delta ferrite in both coatings at approximately 4%. Despite increased cooling rate in the laser process, the differences in the chemical compositions, lead to the same percentage of delta ferrite in both cases. As discussed earlier, dilution is greater in the PTA process due to a higher amount of heat input during the process. The dilution was also measured on the top layer deposition in both cases and the results were: 4% for the laser cladded sample 9% for the PTA welded sample. The dilution is also illustrated in Fig. 14, by using the line analyse from the substrate through the coating. The hardness has been measured and a higher hardness was obtained in the laser cladded sample, 166 HV10, compared to the PTA laser welded sample, 150 HV10. The difference is due to the different cooling rates between the two methods but also as a result of composition transformation due to higher dilution in the PTA sample. The impurities in the substrate can influence the properties of the final coating even in small quantities. A 316L coating was applied on the low Carbon steel component of two different qualities: one with 0,28%Cu and another one containing 2% Cu. The dilution was measured in both cases and following results were obtained: - Low C steel 0,28%Cu dilution 5% - Low C steel 2%Cu dilution 11% This proves that even minor variations in trace elements in the substrate can result in different properties of the final coating. Si Ni Fe Cr Mn Mo Conclusions Despite following identical setup procedure, the final quality of the clad is dependent on several factors: Material used for cladding on component Component geometry Component size Size & number of impurities in the substrate The geometry of the component can influence the cladding pattern needed in order to get a good quality clad The final structure of the clad is a function of the cooling rate, which is dependent on the component size for the same process parameters A finer structure is obtained when laser cladding process is used compared to the PTA process. This
leads to higher hardness for the laser coating versus the PTA coating. Delta ferrite formed during the welding process is critical in order to obtain a crack free structure Dilution can be influenced by the chemistry of the substrate. Acknowledgments The author would like to thank Mr. Kari Westerling and Mrs. Barbara Maroli for their vital contribution to the successful outcome of this work. References [2] Kampanis, Nicholas and Hauer, Ingrid. Propeller Shaft Repair for a Large Ferry with the Aid of Laser Cladding, Surabaya, Indonesia : s.n., 2010. ICSOT Conference. (Roussakis SA, Hellas, Höganäs AB, Sweden). [3] Li, Zhen, Influence of Cold Work and Grain Size on the Pitting Corrosion Resistance of Ferritic Stainless Steel, Seattle : Minerals, Metals and Materials Society/AIME, 2010. TMS 2010 Annual Meeting and Exhibition. [4] Carvalho, Felipe Leal, Influence of Austenitic Grain Size on the Stress Corrosion Cracking Resistance of Steel Applied in Sour Service Environments, Brasil : 61st Annual ABM International Congress, 2006. [5] http://www.migweld.de/english/home. [1] ASM Speciality Handbook, Stainless Steels. ASM International The Materials Information Sociaty