Corrosion Resistance of ASSAB Stavax ESR Stainless Steelby Heat and Cold Treatment

Lee-Long Han1, Chun-Ming Lin1,+ and Yih-Shiun Shih2

1Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology,No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C.2Department of Mechanical Engineering, National Taipei University of Technology,No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C.

This study compared cryogenic treatment, ultra-cryogenic treatment, and high-low temperature tempering treatment using ASSAB StavaxESR, and conducted the following analyses of the prepared specimens: (1) analysis the structure by X-ray diffraction, (3) analysis of the textureof the processed specimens by optical microscopy, (2) using a hardness tester to analyze the changes in hardness of specimens processed bycryogenic and heat treatment, (3) analyzing the hydrophilicity and hydrophobicity by the water contact angle test, and (4) analyzing corrosionresistance by the corrosion resistance test. The experimental results showed that cryogenic temperature affects the amount and shape of thecarbides, which are significantly reduced in cases of high-temperature tempering. The water contact angle test analysis showed that the coatingfilm’s water contact angle performance is the best, followed by specimens of ultra-cryogenic treatment, specimens of cryogenic treatment, andspecimens of traditional heat treatment. It was found that cryogenic treatment can increase polishability, and empower the specimens with goodmold release performance. The coating-film corrosion resistance test showed that ultra-cryogenic treatment and cryogenic treatment can improvecorrosion resistance; however, the performances of specimens by traditional heat treatment were the worst.[doi:10.2320/matertrans.M2012377]

(Received November 8, 2012; Accepted February 18, 2013; Published April 5, 2013)

Keywords: corrosive current, cryogenic treatment, corrosion resistance

1. Introduction

In a working environment of high temperature, highpressure and high stress, the surface of a mold is vulnerableto the formation of oxides, and adhesion of plastic materials.The repeated use of the mold in this situation directly affectsthe quality of plastic components, leading to a shortenedmold service life because of wear and tear caused byadhesion on the mold surface. In particular, the injectionmolding temperature is in the range of 200­400°C, and someplastic materials can easily produce acidic materials at hightemperatures. The material feeding tube and mold of theinjection molding machine is challenged by wear and tear athigh temperatures and corrosion.1) Issues regarding increaseddye life, reduced maintenance costs, and labor waste fordowntime maintenance have become important.

In the case of the Martensite stainless steel mold,2) becauseof its excellent corrosion resistance and overall hardeningproperties, good ductility and tenacity, outstanding abrasionresistance and polishing performance, and its exceptionalcorrosion resistance,3) the optical mold cavity surface remainsas bright as the original after long-term use.

2. Experimental

2.1 Preparation of specimensThis study used ASSAB Stavax ESR stainless steel to

prepare research specimens, with its elemental compositionas shown in Table 1. The specimens underwent temperingheat treatment at 250 and 560°C, and the specimens ofcryogenic and ultra-cryogenic treatment were tempered at250 and 560°C, after being treated at ¹160 and ¹80°C. All

specimens were polished to a mirror-level before conductingexperiments. Prior to conducting experiments, we measuredthe surface roughness of the polished specimens using an¡-step profilometer and determined the surface roughnessvalue to be Ra0.01 µm.

The numbering of the specimens in this study is shown inTable 2, with the process shown in Figs. 1(a), 1(b) and 1(c).

2.2 Analysis of the light diffraction instrumentThis experiment used the XRD-6000, SHIMADZU X-ray

diffraction instrument at a low grazing angle to analyze thecrystal structure and lattice direction of the coating film.

The X-ray light emitting source was the copper target andthe wavelength was ­ = 0.15406 nm, the current was 30mA,the power voltage was 40 kV, and the incident angle waslow at 2°, the diffraction angle range was 20­50°, and thescanning speed was 1°/min.

2.3 Hardness testHardness is representative of material resistance to plastic

deformation. This study conducted the Rockwell hardnesstest using the instrument typed Mitutoyo ARK-600 tomeasure three points of the specimen. The hardness was

Table 1 ASSAB Stavax ESR stainless steel elemental composition(mass%).

Element mass%

Fe 84.32

C 0.38

Si 0.9

V 0.3

Cr 13.6

Mn 0.5

+Corresponding author, E-mail: lcd@mail.tyai.tyc.edu.tw, GraduateStudent, National Taipei University of Technology

Materials Transactions, Vol. 54, No. 5 (2013) pp. 833 to 838©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

represented by indentation depth. The indenter for hardenedsteel and other hard materials was a diamond cone with avertex angle at 120° and tip radius at 0.2mm. The hardnessmeasurements were represented by HRC marks.

2.4 Water contact angle testThe contact angle is the angle between the tangent lie of

the droplet shape and the solid surface. This crossover point

is in contact with air, liquid and solids, as shown in Fig. 2.When the condensed solid­liquid phase is in contact with theother phases, it produces physical and chemical interactions,called surface energy or interface energy, which is the energythat damages surface bonding. This experiment measuredthe contact angle of deionized water. When the contact anglemeasurement was greater, the material was more hydro-phobic; otherwise, the material was more hydrophilic.3)

2.5 Corrosion testThis study used polarization corrosion testing to measure

the surface-corrosion resistance of materials processed byheat treatment and cryogenic treatment. The experimentconditions were at room temperature, with silver chlorideas the reference electrode, and platinum (Pt) electrode as theauxiliary electrode. To simulate the marine environment,the testing solution was 3.5mass% NaCl, and the mainoperating parameters included a scanning rate at 1mV/s,an initial potential at ¹0.8V, a termination potential at+0.1V, and a scanning area at 0.785 cm2. The electrochem-ical corrosion test determined the corrosion potential (Ecorr)and corrosion current (Icorr) of heat treatment, and usedcryogenic treatment to determine the corrosion resistanceperformance.4)

3. Results and Discussion

3.1 Microstructural observationASSAB Stavax ESR is a high-chromium alloy tool of

stainless steel in the same category as martensitic stainlesssteel. The experimental specimens were processed by generalheat treatment, ultra-cryogenic treatment, and cryogenictreatment. In this experiment, the specimens were groundand polished to a mirror surface; the surface of the specimenwas corroded by picric acid, hydrochloric acid, and alcoholsolution before using an optical microscope (OM) to observethe surface microstructure. The experimental results showed

(a)

(b)

(c)

Fig. 1 (a) Cryogenic treatment process (D1, D2, D3, D4), (b) heattreatment process (A1, A2), (c) cryogenic treatment parameter.

Fig. 2 The surface tension analysis when a liquid is on a solid surface.

Table 2 Specimens treatment and No.

Specimens No. Specimen treatment

A1 vacuum hardening + tempering 250°C

A2 vacuum hardening + tempering 560°C

D1 vacuum hardening + ultra cryogenic ¹160°C + tempering 250°C

D2 vacuum hardening + ultra cryogenic ¹160°C + tempering 560°C

D3 vacuum hardening + cryogenic ¹80°C + tempering 250°C

D4 vacuum hardening + cryogenic ¹80°C + tempering 560°C

L.-L. Han, C.-M. Lin and Y.-S. Shih834

that ultra-cryogenic treatment and cryogenic treatment canincrease the precipitation of carbides, which can increasewear resistance performance.5,6)

In tempering experiment A1, martensite bulk iron wastempered, where a few carbides (Cr,Fe)23C6 had precipitatedalong the interface, as shown in Fig. 3(a). In A2 tempering,the main products were tempered martensite bulk iron andcarbides. The amount of carbides (Cr,Fe)23C6 were signifi-cantly less, as shown in Fig. 3(b) because its structure ismore delicate and homogenized after cryogenic treatment.With decreasing cryogenic temperature, it was shown that theamount of carbides (Cr,Fe)23C6 increases, and the shape tendsto be spherical. After high-temperature tempering, carbides(Cr,Fe)23C6 tended to concentrate on the boundary innucleation to form small indentations or more complextree-like arrays. These types of precipitations are harmful tomechanical properties.7,8) When the cryogenic temperaturedecreased, it condensed into a basic structure, as shown inFigs. 3(c) and 3(d). D1 and D2 were more significant, asshown in Figs. 3(e) and 3(f ).

3.2 X-ray diffraction analysisThis experiment conducted XRD analysis on the surface of

specimens processed by heat treatment with incident anglesin the range of 15 to 50°. The residual austenite, after lowtemperature tempering, had diffraction peaks at the crystal-line surfaces (111), (220) and (311). With decreasingcryogenic temperature, the diffraction peak (111) becamegradually flat; this indicated that the remaining austenitebecame significantly less because of cryogenic treatment.The diffraction peak of crystalline surface (110) was causedby the chromium, as shown in Fig. 4. No diffraction peak ofthe remaining austenite, after high-temperature tempering,was observed because the remaining austenite has nearlycompletely been transformed into tempered Martensite.9,10)

3.3 Hardness testIn this experiment, the Rockwell hardness tester was

employed to measure the hardness of the specimensprocessed by heat and cryogenic treatment. Each specimenwas measured by obtaining the average value of themeasurements of five points to avoid the regional effectsof the samples. The experimental results are shown inTable 3. The experimental results show that the hardnessvalues of specimens D1, D3, A1, D2, D4 and A2 arerespectively 50, 50, 48, 42, 42 and HRC 41. The hardnessof the material processed by the cryogenic treatment hasimproved slightly.

3.4 Water contact angle testThis experiment used deionized water droplets to measure

the water contact angle of the specimens to determine the

(a) (b) (c)

(d) (e) (f)

Fig. 3 Metallographic structures. (a) 250°C tempering, (b) 560°C tempering, (c) cryogenic temperature ¹80°C tempering 250°C,(d) cryogenic temperature ¹80°C tempering 560°C, (e) ultra-cryogenic temperature ¹160°C tempering 250°C, (f ) ultra-cryogenictemperature ¹160°C tempering 560°C.

Fig. 4 XRD diffraction analysis diagram. A1 is tempering at 250°C, D3 iscryogenic treatment ¹80°C tempering at 250°C, D1 is ultra-cryogenictreatment at ¹160°C tempering 250°C.

Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat and Cold Treatment 835

material hydrophobic or hydrophilic performance. A greatercontact angle represents better hydrophobic performance; andthus, the material mold release performance is improved. Theexperimental results are shown in Table 4. Specimens D1 andD2 can effectively reduce the remaining austenite to obtainan even martenite structure with large amounts of carbideprecipitations in spherical shapes. Hence, it can betterimprove structural homogenization to easily obtain a smoothsurface, and effectively improve the hydrophobic perform-ance, as shown in Figs. 5(c) and 5(d). The structures ofspecimens D3 and D4 are relatively coarser, the carbides arefewer and have a sharper shape, and the hardness is relativelylower. Hence, its hydrophobic performance is lower than thatof the specimens processed by the ultra-cryogenic treatment,as shown in Figs. 5(e) and 5(f ). However, for specimens A1and A2 because of an uneven structure and few carbideprecipitations, the contact angle is at a minimum, as shown inFigs. 5(a) and 5(b).

3.5 Corrosion testThis experiment used electrochemical testing to explore

the corrosion resistance of materials processed in differentcryogenic and heat treatment temperatures. The corrosionresistance of the specimens can be determined by the

corrosion current (Icorr). A smaller value of Icorr indicatesbetter corrosion resistance. The corrosion potential Ecorr isdetermined by the rates of the anodic and cathodic reactionsof corrosion. The higher Ecorr indicates the lower corrosionrate (higher corrosion resistance) when the cathodic reactiondoes not change. This experiment shows that, when thecryogenic temperature is lower, it can improve lattice size,which renders the structure more delicate and enables it toobtain better corrosion-resistance performance. The corrosionresistance is better in cases of low and medium temperaturetempering than in high-temperature tempering, as the carbideprecipitations at low-temperature tempering are fewer. Thecorrosion resistance of specimens processed by ultra-cryogenic and cryogenic treatment are better than that ofthe high-temperature tempering primarily because thechromium and carbon contents of ASSAB Stavax ESRstainless steel are relatively higher. After the ultra-cryogenicand the cryogenic treatment, the carbides Cr23C6 precipitate;the chromium of these carbides is higher than that of themartenite stainless steel substrates.11) The precipitationsnaturally consume the chromium encompassing the boun-dary, and result in a chromium-insufficient area. After ultra-cryogenic and cryogenic treatment, the carbide precipitationsare significantly greater than that in general heat treatment.

Table 4 Specimens water contact angle.

Specimens No. Water contact angle

A1 80°

A2 74.5°

D1 88.3°

D2 86.6°

D3 83.7°

D4 82.3°

(a) (b) (c)

(d) (e) (f)

Fig. 5 Water contact angle test. (a) 250°C tempering, (b) 560°C tempering, (c) ultra-cryogenic temperature ¹160°C tempering 250°C,(d) ultra-cryogenic temperature ¹160°C tempering 560°C, (e) cryogenic temperature ¹80°C tempering 250°C, (f ) cryogenictemperature ¹80°C tempering 560°C.

Table 3 Experimental specimens hardness measurements.

Specimens No. Average hardness (HRC) Average hardness (HV)

A1 48 484

A2 41 402

D1 50 513

D2 42 412

D3 50 513

D4 42 412

L.-L. Han, C.-M. Lin and Y.-S. Shih836

Thus, when the tempering temperature rises, the diffusion ofchromium is faster than the diffusion of carbon. Because theconcentration of the chromium in the solid solution has notchanged although the carbon concentration significantlyreduces because of the formation of carbides, the chromiumconcentration in the crystal boundary and the crystalchromium is homogenized to reduce the tendency ofcorrosion, and improve corrosion resistance.12­14) For high-temperature tempering of the general heat treatment, thecarbide precipitations result in chromium-depleted regionsthat reduce corrosion resistance performance. For lowtemperature tempering, the homogenization of the chromiumcan lead to better corrosion-resistance performance, as shownin Figs. 6(a) and 6(b). The results of the corrosion potential(Ecorr) and corrosion current (Icorr) were obtained usingCView2 polarization software, as shown in Table 5.

In a neutral solution, the anodic reaction is dissolution ofmetals and the cathodic reaction is oxygen reduction.Figure 6(a) shows the cathodic current at ¹0.6V iscompared, D1 indicate the smallest. If the anodic current at¹0.3V is compared, D3 > A1 > D1 and A2 > D4 > D2.The larger current means lower corrosion resistance. Table 5shows the values calculated using the software. The Ecorr andIcorr of D1 were ¹0.36V. The results show that, throughcryogenic treatment and low-temperature tempering, theinternal tissue of the material can be made more homoge-neous and dense, preventing corrosive media from directlypenetrating the material in a downward manner duringcorrosion.

4. Conclusion

This study used ultra-cryogenic treatment, cryogenictreatment and general heat treatment to compare the sixcombinations of process parameters for STAVAX ESRstainless steel to determine the optimal parameters forSTAVAX ESR stainless steel. The comparison items includedstructure, water contact angle and corrosion resistance.According to the research findings, the following conclusionsare provided.(1) The metallographic test showed that cryogenic treat-

ment can aid in carbide precipitations. With decreasingcryogenic treatment temperature, the amount of car-bides increases and particles become smaller. Thestructure is more delicate and homogenized withdecreasing cryogenic temperature. With rising temper-ing temperature, the amount of carbides increases andcondenses in the substrate structure.

(2) The water contact angle test showed that the specimenwith the maximum contact angle is D1 at 88.3°. Ultra-cryogenic treatment can effectively reduce residualaustenite to obtain a homogenized martenite structure,which leads to large amounts of carbide precipitationsin spherical shapes that improve the structural homog-enizations and smooth the surface to enhance hydro-phobic performance. The greater surface contract anglecan render it easier to release the mold and preventadhesion friction; thus ensuring size accuracy andprolonged mold service life.

(3) The corrosion test showed that the carbon concentrationof various specimens significantly reduces because ofthe formation of carbides, rendering the crystalboundary and crystal chromium homogenized, whichimproves corrosion resistance and reduces the tendencyof corrosion. For high-temperature tempering, theprecipitation of carbides can result in chromiumdepleted areas, which worsens corrosion resistance.For low temperature tempering, the chromium concen-tration homogenization can result in better corrosionresistance performance. Corrosion resistance is relatedto carbide precipitation. Ultra-cryogenic treatment can

(a) (b)

Fig. 6 Corrosion resistance test comparison. (a) 250°C tempering, ultra-cryogenic treatment ¹160°C followed by 250°C tempering,cryogenic treatment ¹80°C followed by 250°C tempering, (b) 560°C tempering, ultra-cryogenic treatment ¹160°C followed by 560°Ctempering, cryogenic treatment ¹80°C followed by 560°C tempering.

Table 5 Polarization measurements of specimens.

Specimens No. Ecorr (V)

A1 ¹0.43

A2 ¹0.44

D1 ¹0.36

D2 ¹0.37

D3 ¹0.43

D4 ¹0.45

Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat and Cold Treatment 837

effectively improve the carbide size and enable thestructure to be more delicate, which improves corrosionresistance capability.

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