Eliminating Delayed Cracks in Deep-Drawn SUS304 Cups Under Wider Range and Lower Magnitude of Blank Holding Forces Using Tin Coated Die


 The effect of TiN coated die on eliminating delayed cracks in deep drawing processes of stainless steel SUS304 cylindrical cups under elevated blank holding forces (BHF) using a commercial lubricant at room temperature is investigated in the experiment. For comparison, the experiment is repeated using an uncoated and finely polished die under the same conditions. The results shows that the crack-free BHF range for the coated and the uncoated dies are 5~10 kN and 12 kN, respectively. Both the magnitude and range of the crack-free BHF are successfully lowered and enlarged by applying TiN coating to the die surface. Lower magnitude and wider range for BHF are preferred in the industries as it is difficult to maintain a high, constant and precise BHF during the deep drawing process using coil springs or die cushions. The elimination of the cracks is mainly due to the decrease in amount of strain-induced martensite resulting from the lower amount of wall thickening, particularly in the valley points along the cup earring profiles. The improved tribological performance by the coating enhances the radial flow of the materials into the die cavity resulting in lower amount of wall thickening. The chance for delayed cracks is reduced with decreasing amount of wall thcikening. Overall, the amount of tensile residual stresses along the outer surface of the cup, particularly in the upper portion is reduced with the coated die due to its low BHF. Therefore, the risk for the cracks is reduced.


Introduction
SUS304 is a metastable austenitic stainless steel containining 18% chromium and 8% nickel. It is the most common type of stainless steels produced in the world. Products such as cookware products, cutlery and medical equipments are made of SUS304. Due to the thermodynamic properties of the unstable austenite phase at room temperature, it tends to transform into a martensitic phase, i.e. straininduced α¢-martensite during deformation, leading to the higher chance for delayed cracks [1]. The cracking susceptibility of SUS304 increases with rising α′-martensite content [2]. Delayed cracking is a mechanism where a subcritical crack produces time-delayed cracking which there is even no external stress applied to the formed components [3]. The formation is attributed to the coexistent of the internal hydrogen content of material, residual stresses, strain-induced α'-martensitic transformation and the chemical composition of the material [4]. The presence of α′-martensite is a necessary prerequisite for delayed cracking to occur in austenitic stainless steels with typical internal hydrogen concentrations (< 5 ppm) [5]. The presence of the tiny amount of hydrogen in the steel is due to the water content in the raw materials or in the furnace gases, during pickling in mineral acids or cathodic cleaning, or during bright annealing. Despite the increase in chance for the cracks, the partial transformation of the austenite microstructure into the martensite microstructure results in excellent combination of strength and elongation [5]. With the combined properties, it is widely used in windshield wipers, brake springs, seat belt retractors and valve springs. It also used as raw material for contact springs, hinge springs and clamp springs in electric connectors. Delayed cracking is also affected by process parameters such as drawing ratios, forming rates and forming temperatures as the strains, residual stresses and phase transformations in the deep drawn cups are greatly in uenced by these parameters [6]. The formation of α'-martensitic is also in uenced by the chemical contents of the steel such as Nickel, Chromium, Carbon, and Nitrogen [7]. This may due to the close relationship between Nickel composition and the stability & stacking fault energy of austenite. Austenitic stability promotes cross slip and, thus reduces planar slip participation in fracture to favour a macroscopically more ductile appearance. Delayed cracking had been prevented with a deep-drawn temperature of 80 °C due to the suppression of the martensitic transformation [6]. In addition, the transformation was also suppressed by the adiabatic heating at high forming rates. Meanwhile, warm hydro-mechanical deep drawing processes at 90 °C had been implemented to control the formation of strain-induced α¢-martensite and increase the limiting drawing ratio of SUS304 cylindrical cups from 2.0 to 3.3 [8]. Applying annealing immediately after the deep drawing is also effective in removing the residual stresses. However, despite the economic disadvantage, it is also very di cult to maintain the close tolerances and ne surface quality aspects of the products during annealing. The magnitude of tensile residual stresses is increased with increasing amount of strain-induced α'-martensitic [9]. Therefore, the concentration of residual stresses in the α'-martensite phase is higher than the one in the austenite phase, leading to increasing risk for the cracks. The tangential residual stresses that attribute the cracks are considered the most vital [10]. The residual stresses can become very large i.e. up to the yielding stress value in the deep drawing process of austenitic stainless-steel blank and cause the appearance of delayed crack [11]. A simulated result indicating the highly concentrated tensile residual hoop stress zones in the valley points along the drawn cup edge after the tool separation stage had been reported [12]. Therefore, the risk for the delayed cracking in these points is higher than others. Raising the drawing ratio also increases the risk for delayed cracking due to the increase in maximum transformed martensitic fractions and residual stresses in the drawn specimens. The severity level of the delayed cracks is increased with increasing drawing ratio of SUS301 cylindrical drawn cups [3]. The risk of delayed cracking was markedly reduced after annealing processes. The hydrogen content of metastable austenitic stainless steel reduced by 1 to 3 wt ppm through heat treatments at 400 °C [5]. T the cup ironing process causes a drastic change in the residual stress resulting in a favourable distribution for preventing the delayed cracking [11]. The decrease in sidewall thickness and change in residual stress distributions of the drawn cups by enhancing BHF during the drawing process [9]. The delayed cracks in the SUS304 drawn cups were successfully eliminated with increasing BHF ranging from 3.6 to 3.9 times the minimum pressure required to suppress the wrinkle aided by nanolubrication using a polished and uncoated die like this study [13]. However, the magnitude and the range of the crack-free BHF were too high and too narrow i.e. 29~31 kN under the same experimental conditions except for the different lubricants. The SiO 2 nanolubrication had been successfully applied in the deep drawing processes of ultra-high strength steel and aluminium alloy cylindrical cups for increase in seizure resistance and in ironing limit [14][15]. Continuous ultrasonic vibration of the SiO 2 nanolubrication is needed to avoid the condensation and the agglomeration of the nanoparticles at the bottom. The deep draw-ability of ultra-high-strength steel cups had been increased with die coatings, particularly with VC-coated die [16]. Due to the excellent adhesion to substrates, resistance to elevated temperatures, hard surfaces (2400HV) to reduce abrasive wear and a low coe cient of friction, TiN coating has been widely used for cutting tool and dies coating [17]. TiCN-based cermet die having ne lubricant pockets is effective in preventing the seizure in the ironing of stainlesssteel drawn cups [18].

Experimental Conditions
The chemical and mechanical properties of the SUS 304 blanks are shown in Table 1 and Table 2, respectively. It is an austenitic steel possessing of a minimum of 18 wt% chromium and 8 wt % nickel or named as 18-8 type stainless steel. It is the most common type of stainless steels used in the world due to its excellent resistant to most oxidizing acids and its ability to withstand all ordinary rusting. Both tensile strength and ductility are high making it particularly suited for both consumer and medical products.  Then, the press compresses the load cell and the press plate, resulting in the movement of the punch into the die. The distance travel of the punch is recorded by a laser distance meter pointing vertically to the press plate. Since the punch and the press plate are moving together during the compression, the punch travel distance is measured. A data logger with a sampling rate of 10 Hz is used to capture the signals from both the load cell and laser distance meter.
Pockets are machined onto the support and the press plates, respectively to rmly hold the load cell from falling during the test.
The photos of the (a) TiN coated, and (b) uncoated drawing dies are shown in Fig. 2. Both dies have the same dimensions with different surface conditions. The uncoated die is made of SKD-11 tool steel. Before each drawing test, both the uncoated die and the blank holder are polished with an orbital sander polisher machine at rotational speed of 4000 RPM using sandpapers of Grade 800 followed by Grade 1000. Finally, a solution of diamond paste with particle size ranging from 2-4 µm in a compatible diamond lubricant is used in the nal stage of the polishing process. However, no polishing is applied to the TiN coated die surface at the beginning of each test. Only the blank holder is polished with the same method as mentioned above. TiN coating is applied to the drawing die made of tool steel by PVD coating process. The measured hardness of the coating is 61 ± 1 HRC, which is approximately 7.5 times higher than the SUS304 blanks. A commercial deep drawing lubricant (ACI PRESSCUT J133) with a viscosity of 285 cSt at 40 °C containing corrosive inhibitors and extreme pressure additives is applied to both drawing dies before each test. The number of cracks and the time taken for its rst appearance in the cups formed with the uncoated die are summarized in Table 3. The time is recorded immediately after completing the test until the formations of all cracks are complete. It clearly shows that the duration for the rst crack increases with increase in BHF. The formation of the cracks is gradually suppressed with increase in BHF. However, the suppression becomes weak when excessive BHFs are applied i.e. from 14 kN and above. A crack-free cup is obtained under BHF of 12 kN. The highest number of cracks is obtained with BHF = 13 kN, followed by 8 kN and others. The longest duration for the rst crack is obtained at BHF of 13 kN. The duration sharply decreases for BHF greater than 13 kN. The data in Table 3  The drawn cups formed with the TiN coated die at elevated BHF is shown in Fig. 8. Wrinkle and tearing around cup bottoms are observed under BHF = 4 kN and 11 kN, respectively. Therefore, the BHF range for successful drawn cups is 5 ~ 10 kN. In comparison with the cups formed with uncoated die, successful BHF range is reduced from 7 ~ 15 kN to 5 ~ 10 kN with the TiN coated die. However, delayed cracks are not observed in the successful drawn cups in its entire BHF range. The crack-free BHF range is successfully lowered and widened from 12 kN to 5 ~ 10 kN by replacing the nely polished uncoated die with the TiN coated die under the same lubrication condition. Lower and wider BHF range is preferred in the industries as it is di cult to maintain a constant and high BHF value with coil springs or die cushion during the process.
The comparison of forming load pro les between cups formed with (a) TiN coated die and (b) Uncoated die is shown in Fig. 9. Since the two dies have the same dimensions, typical bell-shape drawing load pro les are obtained for both dies. The increase in drawing load is very minimum for increase in BHF values in both cases. Peak drawing loads range from 105 ~ 115 kN are obtained around 60% of the total punch travel distance for both dies.
The comparison of peak drawing loads between the TiN coated and uncoated dies at different BHF levels is shown in Fig. 10. Overall, the peak loads for the TiN coated die are lower than that of the uncoated die at the same BHF levels. For coated die, the drawing loads increase with increase in BHF i.e. the frictional forces acting in the blank-die interface increases under higher holding pressure. In contrast, only slight changes in peak drawing loads are observed for uncoated die under elevated BHF. Therefore, the extremepressure performance of the TiN coated die is not as good as the uncoated die under the same lubrication condition. For uncoated die, galling or cold welding tends to form between the surface asperities in the interface between the SUS304 blank and the coated die surface. The galling effect or the continuous forming and breaking of the welds has produced some ne particles that facilitate the sliding motion of the tool over the blank, particularly at high BHF. However, galling is reduced with TiN coating resulting in low drawing load at low BHF. Therefore, the extreme-pressure performance of the coated die is reduced due to absence of galling at the interface.
The comparison of the average heights and the average changes in wall thickness between the drawn cups formed with the TiN coated and uncoated dies at elevated BHF is shown in Fig. 11. Overall, the average cup height formed with the coated die is larger than the uncoated die. The peak and valley heights are slightly increased with increase in BHF for both dies. The heights hit peak values at BHF of 8 kN and 12 kN for the coated and uncoated dies, respectively. Since the only crack-free cup is obtained at the peak height for uncoated die with BHF of 8 kN, increase in cup height, particularly in the valleys is favourable for eliminating the delayed crack. By applying TiN coating to the die surface, larger cup heights are obtained with lower BHF values. Overall, the wall thickness in the valleys for the cups formed with the coated die is smaller than the ones formed with the uncoated die. However, the wall thickness in the peaks for the cups formed with the coated die is at the same level with the uncoated die. Due to constant volume, the materials contributing to the elongated height is originated from the side wall below the cup edge. The average changes in wall thickness in the peaks and the valleys are slightly reduced with increase in BHF for both dies. The average thickness hit minimum percentages at BHF of 8 kN and 12 kN for the coated and uncoated dies, respectively.
The relationship between the average changes in wall thickness and the average heights of the peaks & valleys of the crack-free cups formed with the TiN coated die is illustrated in Fig. 12. Overall, the average wall thickness is increased, and the height is reduced with increase in BHF. The largest height and the smallest wall thickness for both points are obtained at BHF of 8 kN. Under excessive BHF i.e. greater than 8 kN, the tribological performance of the lubricant becomes poor, leading to the reverse trend of both values. However, delayed cracks are not observed in the drawn cups up to the upper BHF limit of 10 kN.
The relationship between the average changes in wall thickness and the average heights of the peaks & valleys of the cups formed with the uncoated die is illustrated in Fig. 13. A similar trend of increase in height and decrease in amount of wall thickening under elevated BHF is obtained with the uncoated die. However, the largest height and the smallest wall thickness for both points are obtained at BHF = 12 kN or 50 % higher than one with the coated die. The minimum BHF for obtaining a crack-free cup with the coated die is 140 % (i.e. reduced from 12 kN to 5 kN) less than one with the uncoated die.
The longitudinal distributions of residual hoop stresses passing through the valley points along the outer surfaces of the crack-free cups obtained from the ring-slitting test are shown in Fig. 14. Overall, the amount of tensile residual stresses of the cup formed with the uncoated die is larger than the ones formed with the coated die in the lower half of the cups due to its high BHF value. For coated die, the increase in BHF reduces both the amount of tensile stresses and the slope of the stress for greater than 80 % of its total height. Low tensile residual stress level with less gradient, particularly in the upper portion along the outer surface of cups is favourable for eliminating the delayed cracks. The favourable residual stress distribution is obtained with the TiN coated die at BHF values much lower than that with the coated die.

Conclusions
The effect of TiN coated die on eliminating the delayed cracks in the deep drawing process of stainless steel SUS304 cylindrical cups under elevated BHF is investigated in the experiment at room temperature.
The results are summarized as follow: 1. The minimum BHF for obtaining a crack-free cup with the coated die is 140 % (i.e. reduced from 12 kN to 5 kN) less than the one with the nely polished uncoated die.
2. The crack-free BHF ranges for the cups formed with the coated and uncoated dies are 5~10 kN and 12 kN, respectively. The range is enlarged by 6 times through the application of TiN coating to the die surface, resulting in a robust process.
3. The elimination of the cracks is mainly due to the decrease in amount of wall thickening and increase in elongated height, particularly in the valley points along the earring pro les of the drawn cups, leading to less amount of strain-induced martensite.
4. The favourable residual stress distribution for eliminating the delayed cracks is obtained with the TiN coated die at BHF values much lower than that with the coated die.

Declarations
Funding -Not applicable

Con icts of interest/Competing interests -Not applicable
Availability of data and material -Available upon request Code availability -Not applicable Authors' contributions -Phoo Yoong Hau was a master student working under the supervision of Tan Chin Joo. Phoo performed the experimtnal works under the guidance of Tan.
Ethics approval -Not applicable   Quarter-section of 3D model of deep drawing process  Comparison of forming load pro les between cups formed with (a) TiN coated die and (b) Uncoated die Figure 10 Comparison of peak loads between TiN coated and uncoated dies at different BHF levels Page 18/21

Figure 11
Comparison of average heights and average changes in wall thickness between drawn cups formed with TiN coated and uncoated dies at elevated BHF

Figure 13
Average changes in wall thickness and average heights of crack-free cups formed with uncoated die Figure 14 Longitudinal distributions of residual hoop stresses passing through valley points along the outer surfaces of the crack-free cups