The Surface Properties of Plastic Mold Steel Subjected to Nitriding Treatment

Chun-Ming Lin1,+, Lee-Long Han2 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.)

The homogeneous microstructure, precise texture, and weldability of pre-hardened AISI P21 plastic mold steel are ideal for mirror-finishedplastic mold cores and cavities. This study applies a nitriding heat treatment and oxynitriding to AISI P21 plastic mold steel, using variousprocessing parameters to determine the influence that nitriding treatment has on the surface characteristics. Regarding the prepared specimen, anoptical microscope was used to analyze the microstructure and thickness of the nitride layer, X-ray diffraction (XRD) was performed to examinethe structure, a 3D surface profile meter was employed to analyze the surface morphology and roughness, and polarization corrosion wasconducted to assess the corrosion resistance. A Rockwell C test was also applied to examine the hardness. The results show that applyingnitriding treatment to AISI P21 plastic mold steel can effectively improve the surface hardness, and the addition of an oxynitride surface layerincreases the corrosion resistance. The nitrogen element under the surface layer, which although relatively hard is brittle, produces ferrousnitrides such as Fe3N and Fe4N that maintain the precise shape of the plastic mold and extend its service life.[doi:10.2320/matertrans.M2012386]

(Received November 20, 2012; Accepted January 25, 2013; Published March 8, 2013)

Keywords: gas nitriding, oxynitriding, surface roughness, corrosion current

1. Introduction

With the continuous improvement in mold technologiesdriven by industry development and market demands, 3Celectronic products are undergoing rapid changes. Inaddition, the demand for light, thin, short, small, andmirror-finished products continues to increase, and therequirements for plastic injection products are becomingstricter. AISI P21 mold steel possesses excellent ductility andsurface hardness, which is unchanged after electric dischargemachining. The homogeneous structure, good optical rotationand high corrosion resistance of AISI P21 mold steel areideal for precision texture compression. Furthermore, AISIP21 mold steel possesses good machinability and is suitablefor surface engineering applications such as nitrogenquenching, chromium plating and titanium plating. As acold mold steel material with high ductility and hardness,AISI P21 mold steel is suitable for high-accuracy plasticmolds.1)

This study applies various gas nitriding treatments toAISI P21 mold steel and tests the conditions of molds andthe damage sustained from corrosion. The objectives ofthis study were to increase the hardness and corrosionresistance of AISI P21 mold steel, reduce molds deteriorationto extend their service life, and decrease production costs.Therefore, this study analyzes the application of AISI P21mold steel to increase its applications in the domestic moldmarket.

2. Experiment

In this paper, a surface modification treatment is applied toAISI P21 mold steel using gas nitriding technologies withvarying processes.2­5)

This study analyzes the influence that gas nitridingtreatment has on the microstructure and mechanical proper-ties of the specimens, comparing treated specimens withuntreated (blank) specimens. Assessments of the corrosionresistance, hardness, surface roughness and hydrophilicity ofthe nitride layer are crucial for understanding the structureand mechanical properties.

2.1 Nitriding equipment and process parametersAfter subjecting AISI P21 mold steel to mechanical

grinding and polishing, the base material was directlyanalyzed. The following three surface-modifying gas nitrid-ing processes were applied to the base material: (1) gasnitriding and post-oxidation,6,7) (2) gas nitriding for 8 h, and(3) gas nitriding for 16 h. The equipment used was a pit-typegas nitriding furnace. Table 1 shows the treatment conditions,and N1, N2 and N3 denote the specimen number.Figures 1(a), 1(b) and 1(c) show the gas nitriding time-temperature curves for Specimens N1, N2 and N3,respectively.

2.2 Observation of the nitride layer using a metal-lurgical test

The microstructure of the nitride layer was analyzed duringthe nitriding process using the specimen profiles. An opticalmicroscope was employed to observe the specimens afterconducting standard metallurgical experiment proceduressuch as wire-electrode cutting, mounting, grinding and etchpolishing.

Table 1 Parameters of specimens in the nitriding process.

Specimens Temperature (°C) Nitriding time Oxidation time

N1 530° 2 h 40min

N2 560° 8 h 0min

N3 560° 16 h 0min

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

Materials Transactions, Vol. 54, No. 4 (2013) pp. 603 to 608©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

2.3 X-ray diffraction (XRD) analysisAfter the nitriding process, the AISI P21 mold steel

base material and specimens were cut to a thickness ofapproximately 6mm, and an X’Pert PRO MPD X-raydiffractometer produced by PANalytical was used to detectchanges in the constituent phase of the specimen surfaces.The copper target Cu K¡ = 0.154056 nm served as theradioactive source for the surface of the prepared specimens,and the relationship between the incident angle and K¡intensity was used to identify the phase structure and changesin the AISI P21 mold steel following nitriding. The XRDoperating current was 30mA, the voltage was 40 kV, and thescanning range was 2ª = 20­80° at a speed of 3°/min. MDIJade 5.0 software was subsequently employed to compare theresulting XRD diffraction patterns and identify the structureof the nitride layer.

2.4 Hardness testA Mitutoyo Rockwell C hardness testing machine with

a 150 g load was used to perform surface indentation testson the specimens. A diamond cone-shaped indentator (120°top-end circular angle tester with a radius of 0.2mm) wasused to measure the surface hardness of each specimen.Five points were examined during each test, and their meanvalue obtained.

2.5 Surface roughness testA ET-4000A 3D surface profiler produced by Kosaka

Laboratory Ltd. was used to examine the 3D surfaceappearance and roughness of the specimens after nitriding.The apparatus comprised a probe motion detector, actioncontroller, control computer, signal amplifier, monitoringscreen and CCD camera. The test drive speed was set to0.05mm/s, and the measurement distance was 0.2mm. Each

specimen group was measured three times and the meanvalue calculated. The surface roughness measurements wereassessed based on the centerline average roughness (Ra),which was defined as the arithmetic mean of deviations fromthe surface axle centerline.

2.6 Polarization corrosion testThe experiments conducted in this study included a

polarization corrosion test to measure the corrosion resistanceof the nitride layer. For this test, saturated silver chloride(SSC), which served as the reference electrode, and platinum(Pt), which served as the auxiliary electrode, were placedinto a solution at ambient temperature for 1200 s to observechanges in the open circuit potential over time. To simulate aseawater environment, a test solution of 3.5mass% NaCl anda scanning speed of 1mV/s were adopted the primaryoperating parameters. Curves were measured from the initialpotential of ¹0.6V to the final potential of +0.7V, andelectrochemical corrosion testing was conducted in ascanning area of 0.95 cm.2) The obtained polarization curveswere analyzed using CView 2 software to determine thecorrosion potential (Ecorr) and corrosion current (Icorr), whichdenote the corrosion resistance of AISI P21 mold steel afternitriding.8­13)

3. Results and Discussion

3.1 Observation of the microstructure based on thenitride layer profile

Figure 2 shows the microstructural morphology of eachspecimen. The results in the figure indicate that an obviousnitride layer at a thickness of 80, 140 or 180 µm forms oneach specimen. Thus, the effects of nitriding were relativelysignificant. The microstructure of each specimen comprised,

(a) (b) (c)

Fig. 1 Gas nitriding time-temperature curves, (a) Nitriding time-temperature curve of N1, (b) Nitriding time-temperature curve of N2,(c) Nitriding time-temperature curve of N3.

Fig. 2 Microstructure morphology of each test piece profile, including: (a) Microstructure of N1 profile, (b) Microstructure of N2 profile,(c) Microstructure of N3 profile.

C.-M. Lin, L.-L. Han and Y.-S. Shih604

from the top to the bottom, an oxidation layer ¼ nitridinglayer ¼ diffusion layer ¼ base material. Although theduration of nitriding for Specimen N1 was less than 3 h, anitride layer was generated because of the anti-abrasive andanti-corrosive surface oxidation layer. The microstructure ofSpecimen N2 after nitriding exhibited regular size grains andlimited change. Furthermore, the thickness and hardness ofthe nitride layer had increased significantly compared to thatof Specimen N1, indicating that an obvious and positiverelationship exists between the nitriding duration and thenitride layer thickness. The grains in the microstructure ofSpecimen N3 after nitriding were large because of the high-temperature environment and lengthy nitrogen atom accu-mulation period. High temperatures resulted in the precip-itation of superfine carbide, which formed iron-basedmartensites. A nitrogen compound layer that differed fromthe diffusion layer also appeared on the surface of thespecimens. This layer is typically known as the whitenitriding layer, comprises hard ¾ phase Fe2-3N, and is theprimary cause of the high surface roughness of Specimen N3.However, the overall surface roughness of Specimen N3improved significantly, and the nitride layer was thicker thanthat of Specimens N1 and N2.

3.2 XRD analysisThis study used a low angle of incidence for XRD analysis

of the AISI P21 mold steel base material and specimensurfaces following the application of various nitridingprocesses. The analysis results confirmed the phase structureof each nitride layer, and are presented in Fig. 3. As shown inFig. 3(a), the XRD analysis results for Specimen A indicatethat crystallization equaled the ¡ phase structure underambient temperatures and possessed two diffraction peaks at2ª = 44.65 and 64.95°. Using a JCPDS card for comparison,the two diffraction peaks possessed diffraction planes (110)and (200), and (110) was the main characteristic peak with adiffraction angle 2ª = 44.65°. This means that the AISI P21

mold steel base material exhibited an ¡-Fe structure underambient temperatures. In addition, the intensity of alldiffraction peaks in the nitride layer after various nitridingprocesses increased significantly in correlation to thenitriding time. Thus, the thickness of each compound layerincreased. The XRD analysis results for Specimen N1 shownin Fig. 3(b) indicate that although the peaks possessed areduced ¡-Fe structure, they did not disappear completelyafter nitriding. However, using a JCPDS card for comparison,displacement of the diffraction angle was observed. Compar-ing the diffraction peaks with higher values showed that thelayer was a Fe3O4-phase oxidation layer, with four strongdiffraction peaks appearing at 2ª = 30.00, 35.45, 56.95 and62.55°, with the corresponding diffraction planes (220),(311), (511) and (440). The Fe3O4-phase oxidation layerformed during the nitriding and oxidation processes andslowed the abrasion and corrosion. According to XRDanalysis results for Specimens N2 and N3 after nitriding,as shown in Figs. 3(c) and 3(d), the nitride layer had atendency to thicken as the nitriding time increased. SpecimenN2 exhibited three relatively strong diffraction peaks at2ª = 38.15, 41.15 and 43.55°, with the correspondingdiffraction planes of (110), (002) and (111). SpecimenN3 possessed three relatively strong diffraction peaks at2ª = 41.25, 43.65 and 57.45°, with the correspondingdiffraction planes of (002), (111) and (112). A comparisonof the diffraction peaks using a JCPDS card indicated that thelayer was an ¾ phase Fe3N oxidation layer. However, thediffraction angles showed limited displacement because ofthe varying nitriding duration. Based on the XRD analysisresults, under the same nitriding temperature, the diffractionpeaks of ¡-Fe decreased gradually as the nitriding timeincreased. This is because the thickness of the nitride layerincreased with longer nitriding durations. The XRD patternfor Specimen N3 showed diffraction peaks in the whitenitriding layer of ¾ phase Fe3N and the oxidation layer of theFeO phase. However, this condition was not observed for

(a) (b)

(c) (d)

Fig. 3 XRD analysis charts of the base material and each test piece after the nitriding process are as follows: (a) XRD diffraction pattern ofA (base material), (b) XRD diffraction pattern of N1, (c) XRD diffraction pattern of N2, (d) XRD diffraction pattern of N3.

The Surface Properties of Plastic Mold Steel Subjected to Nitriding Treatment 605

Specimens N1 and N2. These results correspond to the whitelayer structure found in Specimen N3, as shown in Fig. 2(c).This white layer increased the surface hardness of SpecimenN3. Formation of the oxidation layer is inferred to haveresulted from the high temperature at the blow-in ofSpecimen N3, leading to the long reaction period betweenthe specimen and air. This reaction generated a number ofFeO phase oxidation layers.14,15)

3.3 Hardness analysisThe hardness test is the method most frequently used to

measure the mechanical properties of a material. Table 2shows the hardness test results, and Fig. 4 shows a hardnesscomparison chart for the nitriding specimens. The surfacehardness of the specimens improved significantly afternitriding. The hardness of Specimen N1 reached HRC 55,and that for Specimens N2 and N3 reached HRC 62 and HRC63, respectively. These values are significantly higher thanthe hardness of the base material at HRC 38. These resultsconfirm the efficient hardness increase of the nitride layerand surface abrasion resistance. The increase in the surfacehardness of Specimens N2 and N3 exceeded that forSpecimen N1. This was because the nitriding time waslonger, and the gas nitriding process generated a white layeron the specimen surface, which resulted from the hard yeteasily desquamated ¾ phase Fe2-3N white nitriding layer.Compound layers in the microstructures were also identifiedusing the specimen profiles, as shown in Figs. 2(b) and 2(c).

3.4 Surface roughness analysisTo determine the surface morphology differences between

the nitrided AISI P21 mold steel specimens, a 3D surfacemeasuring instrument was employed. Figure 5 shows the 3Dsurface profiles for each specimen. The surface coarseness ofAISI P21 mold steel increased after the nitriding process.Specifically, the surface roughness rose from an initial valueof 9 nm for Specimen A, 49 nm for Specimen N1, 66 nmfor Specimen N2 and 97 nm for Specimen N3. This wasprimarily because of the specimens’ placement in a high-temperature environment, and the lengthy accumulation ofnitrogen atoms during the nitriding process. The formationof the nitride layer increased the coarseness of the surfacecrystal lattices and the surface roughness. The surfaceroughness also tended to increase with the nitriding time,indicating that surface roughness is significantly and

Table 2 The surface hardness of each specimen.

Specimens Rockwell hardness (HRC)

A 38

N1 55

N2 62

N3 63

Fig. 4 Comparison chart of the hardness after nitriding process.

(a) (b)

(c) (d)

Fig. 5 3D surface profile morphology of each test piece; namely (a) 3D surface profile morphology of A, (b) 3D surface profilemorphology of N1, (c) 3D surface profile morphology of N2, (d) 3D surface profile morphology of N3.

C.-M. Lin, L.-L. Han and Y.-S. Shih606

positively related to the nitriding time. Figure 6 shows thetest results. The surface roughness of the specimens with thethree nitride layers was ranked according to magnitude asN3 > N2 > N1 > A. The surface roughness of Specimen N1was low and beneficial for removing the coating of plasticproducts. Thus, this specimen is relatively valuable for plasticmold cavity surface treatments.16)

3.5 Polarization corrosion analysisThe electrochemical polarization curve test was initially

used to assess the corrosion resistance of each nitride layerspecimen. The electromechanical test primarily assessed thepotential difference produced by the significant aggregationof ions leading to cell concentration. The corrosion current(Icorr) determined the corrosion resistance of the nitride layer,with a smaller Icorr value indicating a superior corrosionresistance. The corrosion potential (Ecorr) denoted thecorrosion resistance. Thus, a greater Ecorr value indicated ahigher corrosion resistance of the nitride layer. Consequently,the difficulty of corroding the nitride layer can be determinedusing the Icorr and Ecorr values.17,18)

The polarization curve of each specimen during thenitriding process was obtained after nitriding treatment. Thisprocess used the electrochemical formula that employeda 3.5mass% NaCl water solution, as shown in Fig. 7.The corrosion resistance of all specimens after nitridingwas satisfactory, and all passivation zones appeared. Thepassivation zones appeared because gas nitriding contributedto the formation of an anti-corrosive nitride layer of ¾ phaseFe3N on the specimen surfaces. This nitride layer can blockthe NaCl water solution, reducing the corrosion current (Icorr),decreasing the corrosion speed, and limiting the corrosionphenomenon. The passivation zone of Specimen N1 was themost obvious because it exhibited all the anti-corrosionrequirements of the nitriding layer. In addition, the CView 2software was employed to create a tangent curve intersectionto determine the corrosion current (Icorr) and corrosionpotential (Ecorr) of the nitride layer for each specimen, asshown in Table 3. The corrosion resistance of the basematerial (A) was comparatively poor, and based on thevalue obtained from the software conversion, this materialspecimen corrodes easily compared to others in the study.

Fig. 6 Comparison chart of the surface roughness.

Table 3 Icorr and Ecorr values of each specimen.

Specimens Icorr (A/cm2) Ecorr (mV)

N1 1.5022 © 10¹7 ¹0.23816

N2 13.1270 © 10¹7 ¹0.75021

N3 6.5801 © 10¹7 ¹0.36360

A 9.2354 © 10¹5 ¹0.48950

(a) (b)

(c) (d)

Fig. 7 Polarization curve charts of each specimen, include: (a) Polarization curve chart of N1, (b) Polarization curve chart of the N2,(c) Polarization curve chart of the N3, (d) Polarization curve chart of A.

The Surface Properties of Plastic Mold Steel Subjected to Nitriding Treatment 607

Specimens N1, N2 and N3 all exhibited satisfactorycorrosion resistance. The specimens were ranked N1 >N3 > N2 > A according to their corrosion resistance. Thus,Specimen N1 exhibited the highest corrosion resistance, andthe base material possesses a comparatively poor corrosionresistance.

In summary, the corrosion analysis results shown in thevarious figures and tables indicate that Specimen N1possessed the optimal passivation zone, lowest corrosioncurrent, and maximum corrosion potential, thereby providingthe highest corrosion resistance.

4. Conclusion

(1) The XRD analysis results of this study show thatalthough the ¡-Fe peaks in Specimen N1 were reduced,they did not disappear completely after nitriding. Acomparison of the diffraction peaks indicated that thelayer was a Fe3O4-phase oxidation layer. This resultedfrom the nitriding and oxidation processes, and couldreduce abrasion and resist corrosion. The diffractionanalysis of Specimens N2 and N3 following nitridingshowed that the nitride layer tended to increase as thenitriding time increased. The intensity of all diffractionpeaks of the nitride layers also increased, indicating thateach compound layer thickened accordingly.

(2) The hardness test results show that the surface hardnessof the material increased significantly after the nitridingprocess. The hardness of Specimen N1 reached HRC55, and that for Specimens N2 and N3 reached HRC 62and HRC 63, respectively. These values significantlyexceed the hardness of the base material at HRC 38.Thus, the results of this study clearly show that thenitride layer increased the hardness and efficiency of thesurface abrasion resistance.

(3) The surface roughness test showed that the surfacecoarseness of AISI P21 mold steel increased after thenitriding process. The surface roughness exhibited atendency to increase in correlation to the nitriding time.Furthermore, the surface roughness was significantlyand positively related to the nitriding time. The surfaceroughness of the specimens with the three nitridelayers was ranked according to magnitude: N3 > N2 >N1 > A. The surface roughness of Specimen N1 was49 nm.

(4) The polarization corrosion test showed that SpecimenN1 possessed an obvious passivation zone and lowcorrosion current. Therefore, this specimen can effec-tively reduce corrosion speeds and limit the corrosionphenomenon. The corrosion current (Icorr) for SpecimenN1 was 1.5022 © 10¹7. Thus, the nitriding process ofSpecimen N1 achieved the highest corrosion resistance.

A review of the test results leads to the followingconclusion: Applying gas nitriding to the surface of AISIP21 mold steel can effectively improve its mechanicalproperties. Furthermore, the application of oxynitridingprovides the optimum results.

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