optimizedsteelselectionforapplications in plastics … · 2012-05-16 · optimized steel selection...

OPTIMIZED STEEL SELECTION FOR APPLICATIONSIN PLASTICS PROCESSING

C. ErnstEdelstahl Witten-Krefeld GmbH

Research Tool Steels

Oberschlesienstraße 16

D-47807 Krefeld

W. PannesEdelstahl Witten-Krefeld GmbH

Q uality Department Tool Steels

Auestraße 4

D-58452 Witten

AbstractA survey on the material properties and examples for the application of

some precipitation hardening tool steels in plastics processing are presented.The new steel group "Thyroplast PH" offers an alternative tothe conven-tional hardened and tempered plastic mould steels for applications either ashigh-volume tools with improved mechanical and corrosion properties or aslow-volume tools with improved machinability. After solution annealing andaging the steels of this group reach a hardness of up to 42 HRC and showan improved compressive strength. By modifying the chemical composition,especially by lowering the amount of carbon, weldability and electrical dis-charge machinability are significantly improved. Sulphur additions help toadvance machinability.

In detail, the new materials show the following specific properties: Thy-roplast PH 42 FM combines good machinability and high hardness. It is rec-ommended for building mould frames and backup plates with high demandson strength as well as for hot runner systems. By remelting, Thyroplast PH 42SUPRA obtains its high degree of purity which grants excellent polishability

321

322 6TH INTERNATIONAL TOOLING CONFERENCE

and suitability for texturing. Examples for application are plastic injectionand compression moulds. The corrosion-resistant sulphurized steel Thyro-plast PH X FM with improved machinability is used for the production ofmould frames, constructional parts and plastic moulds withlow requirementson polishability which have to show resistance to condensation and cool-ing waters. Thyroplast PH X SUPRA with improved corrosion resistanceand toughness is favoured for building plastic mould inserts which are undercorrosive attack by the plastics to be processed.

Keywords: Plasticmouldsteel, precipitationhardening, heat treatment, hardness, machin-ability, toughness, corrosion resistance, weldability, typical application

INTRODUCTION

Plastics as innovative materials have been conquering moreand morefields of applicationandhave successfully substituted many traditional metal-lic materials. Worldwide in the year 1998 a total amount of more than 150million tons of plastics was produced, a third of it in EuropeFig. 1. Thesubstitution of traditionally metallic compounds, especially in the vehicleindustry, is one of the most important reasons for this rapidgrowth. As anexample, the use of plastics in car building has increased from 7 % to 12% during the past 25 years [1]. Besides of this application many householdarticles, parts of electronic devices such as computer houses, furniture andcontainers for food and beverages are made of plastics.

Plastic products are manufactured by mass production usingthermoplas-tic and duroplastic bulk materials which are processed by injection moulding,blow moulding and compression moulding. The intensive increase in pro-duction and consumption of plastics has also influenced the tool steel marketwith its demand for increasing amounts and good availability of tool steels.The described field of application therefore is an importantmarket segmentfor the producer of these steels. The tool steel suitable forthe different ap-plication in plastics processing have to show specific properties and needcontinual research and development.

In the production of plastic items via extrusion, injection, blowing, com-pressing and also via deep drawing, the mould is of main significance for theefficiency of the process as the costs for the mould influence the productioncosts in a fundamental way. Extremely important for the toolbuilder and thetool user are an economic production of the tools and a high lifetime of themould. On selecting the right plastic mould steel grade fromthe wide variety

Optimized Steel Selection for Applications in Plastics Processing 323

Figure 1. Production of plastics worldwide, in Europe and in Germany.

of steels available the different requirements on the toolswhich depend onthe type of application have to be known and respected. The most importantproperties of plastic mould steels are wear resistance, hardness, corrosionresistance, toughness, polishability, texturizing properties, weldability andmachinability. It is difficult to find these properties combined in one steelgrade as some of them counteract each other.

According to their heat treatment the tool steels suitable for plastic mould-ing can be divided into four main groups which are listed in Table 1. Mostplastic mould steels are delivered in the soft annealed condition with a max-imum hardness of 250 HB equivalent to 850 N/mm2. In this condition, thebest machinability is guarantied. A large disadvantage of these steels is theadditional heat treatment that has to be carried out by the tool builder afterthe machining process. Today more and more large parts, e.g.componentsof car bodies, are made of plastics. The use of case-hardening steels for largetools is not recommended as they cannot provide the requiredproperties overthe whole cross-section. Here the use of quenched and tempered steels is thesolution. They are completely heat treated by the steel producer and reveala strength of approximately 1000 N/mm2 in as-delivered condition. Thegreat advantage is that tools made of quenched and tempered steels do notbear the risk of distortion due to heat treatment after tool building but their


machinability is decreased compared to the steels machinedin soft-annealedcondition.

Table 1. Steels for plastic moulding, an overview

GroupExampleMat.-№.

Delivery condition and deliveryhardness

Heat treatment &hardness

Quenched andtempered steels

1.2311 Q + T, 280 - 325 HB not necessary1.2312 Q + T, 280 - 325 HB not necessary1.2711 Q + T, 280 - 325 HB not necessary1.2738 Q + T, 280 - 325 HB not necessary1.2316 Q + T, 280 - 325 HB not necessary1.2085 Q + T, 280 - 325 HB not necessary

Through-hardenable

steels

1.2343 soft annealed, < 230 HB 46 – 52 HRC1.2379 soft annealed, < 250 HB 58 – 62 HRC1.2767 soft annealed, < 260 HB 50 – 56 HRC1.2842 soft annealed, < 220 HB 58 – 60 HRC1.2083 soft annealed, < 230 HB 50 – 56 HRC1.2361 soft annealed, < 265 HB 50 – 58 HRC

Case hardeningsteels

1.2162 soft annealed, < 210 HB surface 60 HRC1.2764 soft annealed, < 250 HB surface 60 HRC

Nitriding steels1.8521 Q + T, 230 - 290 HB surface 65 HRC1.8550 Q + T, 230 - 290 HB surface 68 HRC

Precipitationhardening

steels

PH 42 SUPRA sol. annealed + aged, 38 – 42 HRC not necessaryPH 42 FM sol. annealed + aged, 38 – 42 HRC not necessary

PH X SUPRA sol. annealed + aged, 38 – 42 HRC not necessaryPH X FM sol. annealed + aged, 32 – 38 HRC not necessary

In this connection, the element sulphur has gained large importance in thepast. Sulphur is added to low alloyed steels and to corrosionresistant steels toimprove machinability as it has a chip breaking effect. If the tool to be buildneeds a lot of machining, the sulphur alloyed grade 1.2312 (40CrMnMoS8-6) for example is an appropriate steel with improved machinability. Dueto the addition of sulphur, inclusions of MnS are formed which on the onehand improve machinability but on the other hand reduce the toughness ofthe steel. Further in the nineties calcium treated, quenched and temperedplastic mould steels without sulphur addition were developed that showedgood machinability and better isotropy of mechanical-technological proper-


ties [2]. The favourable machinability of the calcium treated steels is basedon the formation of a layer consisting of calcium compounds on the cut-ting edge. At high cutting speeds, the calcium compounds become soft andsemiliquid so that the cutting edge is protected with a lubricant, wear pro-tecting layer. Nowadays, calcium treatment for machinability improvementof plastic mould steels is state-of-the-art.

Constantly increasing demands in particular on the efficiency and dura-bility of the plastic moulds applied have motivated Edelstahl Witten-KrefeldGmbH (EWK) to include mould steels into the production programme whichhave a higher hardness in the as-delivered condition compared to conven-tional hardened and tempered steels and which at the same time still showa good machinability Table 1. These materials are of a group of steels withcompletely different heat treatment compared to the quenched and temperedsteels. The EWK short name "Thyroplast PH" characterizes this group ofprecipitation hardened steels which is divided into corrosion-resistant andnon corrosion-resistant steels each with highest degree ofpurity (conditionSUPRA) or additionally improved machinability (conditionFM).

QUENCHED AND TEMPERED STEELS VERSUS PHSTEELS

To meet the demands on the plastic mould steels a heat treatment proce-dure before or after the machining of the tools is necessary.This procedureinvolves changes in the microstructure to achieve certain properties whichare important either for the following manufacturing steps, such as polishingor texturing, or for the application of the tools. In generalthe type of heattreatment depends on the chemical composition of the steelsFig. 2. Carboncontaining, alloyed steels which are mainly used for makingplastic moulds,are quenched and tempered (heating up to hardening temperature, holding atthis temperature and quenching with sufficient speed to achieve a martensitictransformation, followed by one or several tempering steps).

The less commonly known nickel alloyed steels with a low carbon contentneed a precipitation hardening procedure, consisting of a solution annealingto dissolve precipitated particles and of an aging procedure aiming at arenewed precipitation of metallic or intermetallic particles with smaller sizeand fine distribution. Besides of other changes in the material properties an


Figure 2. Classification according to the heat treatment.

increase in tensile strengthRm and in hardness are the results of this heattreatment.

Of the group of quenched and tempered plastic mould steels which do notneed an additionally heat treatment by the tool builder especially the low al-loyed steels with material№1.2311 (40CrMnMo7), 1.2312 (40CrMnMoS8-6), 1.2711 (54NiCrMoV6) and 1.2738 (40CrMnNiMo8-6-4) havefoundwidespread use. These steels are usually applied in a highlytempered con-dition with a hardness between 280 and 325 HB. Due to their lowcontentsof alloying additions they show a high thermal conductivity. A further ad-vantage which helps to decrease the total production costs of a plastic mouldis the good machinability which is achieved either by a calcium treatmentin the steelworks or by additions of sulphur in a range of up to0,10 %. Toobtain a hardness increase by the formation of martensite, these steels arealloyed with carbon. The interstitial carbon is needed for the distortion ofthe crystal lattice on changing from the austenitic to the martensitic phase.It also forms carbides which are precipitated pre-eutectically and during thetempering process. According to the necessity of repair welding or weldingoperations resulting from changes in the design of the tool the hardened andtempered tool steels are welded although the weldability isreduced by theircarbon content. Increasing contents of carbon are responsible for a decreasein weldability as a distinct rise in hardness in the heat-affected zone due to


martensitic transformation takes place during cooling from welding temper-ature [3]. In these hardened areas, cracking of the welding seam is likely tohappen if no additional heat treatment is carried out after welding.

Compared to the described hardened and tempered tool steels, precip-itation hardened plastic mould steels which are aged at temperatures be-tween 480◦Cand 550◦Cshow an increased as-delivered hardness of 38 to42 HRC Fig. 3 and therefore a better wear resistance [4]. The high com-pressive strength and the resistance against surface deformations leads to agood stability of the clamping edges of the mould. Despite oftheir higheras-delivered hardness precipitation hardened steels can still be machinedreasonably. Due to their low carbon contents they show improved weld-ability. In contrast to the hardened and tempered steels thehardness of theunaffected material and the welded area is not very different. The risk ofcracking after welding operations therefore is reduced.

Figure 3. Comparison of hardness, quenched and tempered steel and PH-steel.

NON CORROSION-RESISTANT STEELS OF TYPETHYROPLAST PH 42

The steels of type Thyroplast PH 42 are medium alloyed materials onthe basis of 3 % nickel, 1,5 % manganese, 1 % aluminium and 1 % copper.Thyroplast PH 42 FM as an alternative to the well-known steel1.2312 ad-ditionally is alloyed with more sulphur to promote machinability. The steelThyroplast PH 42 SUPRA which always is delivered in remeltedcondition, is


an alternative to the steels 1.2311 and 1.2738, combining a high as-deliveredhardness of 38 to 42 HRC with excellent polishability Table 2.

Table 2. Steels of type Thyroplast PH 42

Standard grade Alternative∗Chemical composition in %

C Mn Ni Cu Al S

THYROPLAST 2311 PH 42 SUPRA 0.15 1.5 3.0 1.0 1.0 0.003THYROPLAST 2312 PH 42 FM 0.15 1.5 3.0 1.0 1.0 0.100

∗ Higher hardnessImproved weldabilityImproved polishability (SUPRA)Improved machinability (FM)

To investigate one of the most important properties of a plastic mould steelwhich is machinability, drilling tests with twist drills made of high speedsteel 1.3207 (HS10-4-3-10) without cooling were carried out. The testswere based on Stahl-Eisen-Prüfblatt SEP 1161 ("Tool life test at elevatedtemperatures"). As a criterion for tool life, the failure ofthe cutting edgeswas employed. In Fig. 4 the tool life versus the cutting speedis shown.For both steels without sulphur addition, steel 1.2311 and Thyroplast PH

Figure 4. Machinability investigated in drilling test.


42 SUPRA, a similar tool life was determined although the hardness ofThyroplast PH 42 SUPRA is approximately 10 HRC higher. Drilling of thesteels with sulphur addition, steel 1.2312 and Thyroplast PH 42 FM, leads toa longer tool life. Here especially the conduct of Thyroplast PH 42 FM hasto be emphasized as it also has a higher hardness compared to the quenchedand tempered steel 1.2312.

The ideal applications for the sulphur-alloyed steel Thyroplast PH 42 FMare mould frames, base plates and tool components with an increased amountof machining and at the same time high demands on stability aswell as onfirmness of the edges. An example for such tools are manifold blocks for hot-runner systems in injection moulding Fig. 5. On the other hand, ThyroplastPH 42 SUPRA shows a minimal sulphur content and on principle is suppliedin remelted condition which grants an outstanding polishability. This plasticmould steel therefore especially can be applied for small and medium sizeplastic moulds and mould inserts with high demands on polishability.

Tools for the processing of plastics are occasionally repair welded orwelding operations are carried out which result from changes in the designof the tool. The weldability of tool steels which are used forthe produc-tion of plastic moulds likewise is an important property. Toinvestigate theinfluences of a welding procedure on the hardness and the microstructure,small flat specimens of the steels Thyroplast PH 42 SUPRA and 1.2311were produced, their condition being precipitation hardened or quenchedand tempered, respectively. By melting a seam via the TIG-process with anamperage of 150 A, a welding process without preheating of the specimenswas simulated. The hardness in the cross-section of the molten and heat-affected zones was determined by Vickers hardness measurements Fig. 6.

In steel 1.2311 a new hardened zone can be detected which is caused bythe high temperatures and results from the formation of martensite. Thiszone is followed by a tempered zone with a significantly decreased hardnesscompared to the unaffected material. To avoid cracking and problems duringpolishing and structural etching, a heat treatment is necessary by all meansto adjust the different hardnesses and microstructures. Incontrast to that,the steel Thyroplast PH 42 SUPRA only shows little variationin hardnessafter welding. In the heat-affected zone, the welding temperatures lead to an"overaging" effect which causes a slight decrease in hardness. Altogether,the differences in hardness between the unaffected material, the heat-affected


(a) Manifold block (b) Injection mould

(c) Blowing mould (d) Injection nozzle

Figure 5. Typical applications of steels Thyroplast PH.

zone and the molten zone are clearly much lower compared to steel 1.2311,so that a post-weld treatment is not necessary.

A concluding summary of the properties of the precipitationhardenendplastic mould steels Thyroplast PH 42 SUPRA and Thyroplast PH 42 FMcompared to the hardened and tempered standard tool steels 1.2311 and1.2312 is given in Table 3.


Figure 6. Simulated welding test, hardness after welding.

Table 3. Comparison of properties

hardnesswear-

resistancepolish-ability

photo-etchability

machin-ability

weld-ability

Thyroplast 2311 + + + + + + + +Thyroplast PH 42 Supra + + + + + + + + + + + + + +Thyroplast 2312 + + + O + + +Thyroplast PH 42 FM + + + + + O + + + + + +

CORROSION-RESISTANT STEELS OF TYPETHYROPLAST PH X

Two further developments concerning corrosion-resistantplastic mouldsteels are worth mentioning. As an alternative to the carbonalloyed toolsteels 1.2316 (X36CrMo17) and 1.2085 (X33CrS16) which are generallyused in their as-delivered condition with a hardness of around 300 HB twoprecipitation hardened plastic mould steels are now available Table 4. Sim-ilar to the non corrosion-resistant steels described above, these steels are a


remelted steel with increased polishability named Thyroplast PH X SUPRAand a sulphur-alloyed steel with improved machinability, Thyroplast PH XFM.

Table 4. Steels of type Thyroplast PH X

Standard grade Alternative∗Chemical composition in %

C Mn Ni Cr Cu, Nb S

THYROPLAST 2316 PH X SUPRA 0.05 0.5 4.5 15.0 + 0.003THYROPLAST 2085 PH X FM 0.04 1.3 0.5 11.5 + 0.100

∗ Higher hardnessImproved corrosion resistanceImproved toughness (SUPRA)Improved machinability (FM)

Thyroplast PH X SUPRA is a corrosion-resistant tool steel for plasticmoulds and mould inserts which have to meet highest demands on corrosionresistance and hardness. Its chemical composition is basedon the Americansteels according to AISI 15-5 PH and AISI 17-4 PH, steels witha carboncontent below 0,07 % that originally were used in applications where highstrength, toughness and moderate corrosion resistance were required. Thesesteels especially were used for building components for theaerospace, nu-clear and naval industry [5, 6]. Accordingly Thyroplast PH XSUPRA offerssome advantages compared to standard quenched and temperedsteels suchas 1.2316: a higher hardness, an improved polishability, better weldabilityand corrosion resistance as well as an excellent toughness.

Properly heat treated, that means solution annealed at 1020◦Cand aged ataround 500◦C, Thyroplast PH X Supra consists of a lath martensite matrixinto which copper- rich epsilon-phase and fine Nb-containing carbides areembedded [7]. The aging results in a marked hardness and strength increaseabove that observed in the solution annealed condition. Values of 40 HRCequivalent to 1260 N/mm2 are reached. This strength increase is not accom-panied by any significant decrease in the tensile ductility.Moreover, anotherbig advantage of Thyroplast PH X SUPRA compared to the standard toolsteel 1.2316 is its improved toughness at higher hardness levels [8]. Typi-cal values that were determined in impact bending tests applying unnotchedspecimens are distinctly higher than 250 J Fig. 7.


Figure 7. Toughness of steel Thyroplast PH X SUPRA.

Especially the corrosion resistance of Thyroplast PH X SUPRA is dis-tinctly improved compared to steel 1.2316. The results fromdipping tests indifferent corrosive agents such as acetic acid, nitric acid, hydrochloric acidand sulphuric acid are included in Fig. 8 which shows the gravimetricallydetermined mass loss rate of the specimens. In dependence onthe acid ap-plied, the mass loss rate of Thyroplast PH X SUPRA is one or more powersof ten lower than the rate of steel 1.2316. Two of the reasons for ThyroplastPH X SUPRA’s improved corrosion resistance are its higher chromium plusnickel content and the lack of coarse carbide particles. Also, a passivationeffect of copper has been reported in literature for 15-5 and17-4 PH steels[5], as copper dissolves as Cu+ and Cu++-ions, forming stable corrosionproducts like Cu2O on the steel surface. Thus the pitting corrosion resistanceis improved by inhibiting the anodic reaction.

Thyroplast PH X FM is a new sulphur-alloyed steel with resistance againstcondensation and cooling waters which offers an alternative to the quenchedand tempered steel according to mat.-№. 1.2085. Typical for Thyroplast PHX FM is its much lower carbon content of 0,04 % and lower chromium con-tent of 11,5 % compared tosteel 1.2085, resulting ina higherchromium/carbon


Figure 8. Corrosion resistance of Thyroplast PH X.

ratio. As can be seen from the results of dipping tests in artifical sea water,this ratio has a positive effect on the corrosion resistanceFig. 9. Also, re-sistance against humid conditions, as was tested in an atmospheric chamberwhere dry and humid conditions were alternated with each other for twoweeks’ testing time, was found to be improved.

Figure 9. Sample surfaces after test in artificial sea water.


Due to its sulphur content, the steel shows improved machinability. Thisproperty was investigated in drilling tests with twist drills made of high speedsteel 1.3343. In this test, holes with a depth of 24 mm were drilled (cuttingspeed 12 m/min, feed 0,12 mm/r, under cooling). The wear at the cut-ting edges as well as the flank wear were determined using a microscopicalmeasuring system. The steels 1.2085 in quenched and tempered condition(hardness 325 HB) and Thyroplast PH X FM in precipitation hardened con-dition (hardness 380 HB) were used as base materials for drilling. Figure 10shows the highest and lowest wear that occurred at the edges and flanks. Inspite of its higher hardness, wear at the cutting edges on drilling steel Thy-roplast PH X FM is comparable to the wear that appears when steel 1.2085with lower hardness is drilled. The flank wear is significantly reduced. Asimilar behaviour was found in milling tests where gravers made of steel1.3207 were applied as tools. Here the wear of the cutting edges was twiceas high for the steel 1.2085 after a cutting length of 2,5 m compared to thesteel Thyroplast PH X FM.

Due to its properties, Thyroplast PH X FM in particular is suitable formaking mould frames for multicavity dies with increased demands on cor-rosion resistance (marine climate, cooling water, condensation water) andmachinability. A comparison of the properties of the new precipitation hard-ened steels is given in Table 5.

Figure 10. Wear of tools in drilling tests.


Table 5. Comparison of properties

hardnesswearresis-tance

corrosionresis-tance

polish-ability

machin-ability

weld-ability

Thyroplast 2316 + + + + + + O +Thyroplast PH X Supra + + + + + + + + + + O + +Thyroplast 2085 + + + + + + +Thyroplast PH X FM + + + + + + + + + + +

SUMMARY

The new steel group "Thyroplast PH" which shows a higher tensile andcompressive strength in as-delivered condition offers an alternative to theconventional quenched and tempered plastic mould steels. After solutionannealing and aging the four steels from this group reach a hardness of upto 42 HRC, a heat treatment after machining by the tool builder is not nec-essary. By modifying the chemical composition, especiallyby lowering theamount of carbon, weldability and electrical discharge machinability weresignificantly improved. Sulphur additions help to advance machinability.

In detail, the new materials show the following specific properties: Thy-roplast PH 42 FM combines good machinability and high hardness. It isrecommended for building mould frames and backup plates with high de-mands on strength. By remelting, Thyroplast PH 42 SUPRA obtains itshigh degree of purity which grants excellent polishabilityand suitability fortexturing. Examples for application are plastic injectionand compressionmoulds. The corrosion-resistant sulphurized steel Thyroplast PH X FMwith improved machinability is used for the production of mould frames,constructional parts and plastic moulds with low requirements on polisha-bility which have to show resistance to condensation and cooling waters.Thyroplast PH X SUPRA with improved corrosion resistance and tough-ness is favoured for the production of plastic mould insertswhich are undercorrosive attack by the plastics to be processed.

REFERENCES

[1] R. BAUN, „Die Vielfalt der Werkstoffe – ein Garant für denFortschritt im Automobil-bau, in Werkstoffe in der Automobilindustrie (1999), p. 4 – 13.


[2] H.-J. BECKER, E. HABERLING, P. OTTER, „Eigenschaften calciumbehandelter Kun-ststoffformenstähle, Thyssen Edelstahl Techn. Ber. 15 (1989) 2, p. 95 – 101.

[3] K. RASCHE, „Schweißen von Kunststoffformenstählen", Schweizer Maschinenmarkt(1981) 45, p. 74 – 78.

[4] C. ERNST, „Substitution des Legierungselementes Kupfer in aushärtbarenWerkzeugstählen für die Kunststoffverarbeitung", Dissertation Ruhr-UniversitätBochum (2001).

[5] U. KAMACHI MUDALLI et al, „Localised corrosion behaviour of 17-4 PH stainlesssteel", Materials Science and Technology (1990) 6, p. 475 – 481.

[6] H. KREBS, "Entwicklungsstand und Tendenzen bei ausscheidungshärtbaren Edel-stählen", Maschinenmarkt 103 (1997) 13, p. 46 – 49.

[7] H. J. RACK, D. KALISH, "The strength, fracture toughness, and low cycle fatiguebehaviour of 17-4 PH stainless steels", Metallurgical Transactions (1974) 5, p. 1595 –1605.

[8] C. ERNST, „Neue aushärtbare Werkzeugstähle", Form + Werkzeug 1 (2002), p. 51 –53.

optimizedsteelselectionforapplications in plastics … · 2012-05-16 · optimized steel selection...

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