NEW OPTIMISED MANUFACTURING ROUTE

NEW OPTIMISED MANUFACTURING ROUTEFOR PM TOOL STEELS AND HIGH SPEED STEELS

C. Tornberg and A. FölzerBöhler-Uddeholm Powder Technology

Mariazeller Strasse 25

A-8605 Kapfenberg

Austria

AbstractThe absence of non metallic inclusions in tooling materialshas been a

driving force for the development of new processing technologies to obtaina top quality starting material for tool manufacture.

In the PM-route for tool steels and high speed steels the treatment ofthe melt before atomisation is of course one important factor to control theinclusion level in the steel, but also the further handling and consolidation ofthe atomised powder are crucial in reaching a PM compact withuniform andreliable properties.

This paper will review the considerations behind the designof an optimisedprocess route for PM Tool Steels and High Speed Steels. The route has beenimplemented in the new Böhler-Uddeholm PM plant in Kapfenberg. Productand process experiences are reviewed.

INTRODUCTION

Over the years since the commercial start of PM Tool Steels (TS) andHigh Speed Steels (HSS) in the early 70s it has been evident how improvedquality of the starting material for tool manufacture in particular with respectto non metallic inclusions can enhance tool life. In high carbide containingtool steels fracture occurs without any noticeable prior plastic deformation.The stress level necessary for causing fracture is basically dependent onthe size and location of the initiating defect. Such defectsare primarily

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364 6TH INTERNATIONAL TOOLING CONFERENCE

large carbide clusters or non metallic inclusions. The harmful effect of largecarbide clusters has been eliminated by powder metallurgical processingleaving non metallic inclusions as the main threat to the life of the tool.

Figure 1.

For this reason the development work in the PM tool steel sector has forthe last decades been focused on minimizing the occurrence of inclusions.Another favourable effect of PM processing is the uniformity of propertiesin the consolidated PM steel as opposed to conventionally produced ingotmaterial where segregation during solidification and the ingot shape cre-ate inhomogeneous structures and properties. The uniformity of the PMsteel result in for example less distortion of the tool afterheat treatment.Development work therefore has also been directed towards optimising thehomogeneity of the PM compact in order to further improve dimensionalstability and uniform mechanical properties. However different PM pro-cessing routes give various results in terms of homogeneityof the structureafter hot consolidation as will be shown later in the text.

The traditional production route for PM tool steels has for long been

powder manufacture by nitrogen gas atomisation of a prealloyed melt,

encapsulation of the thus produced spherical powder in metal contain-ers,

New Optimised Manufacturing Route for PM Tool Steels and High Speed Steels365

Figure 2. ESH tundish with electromagnetic stirrer (EMS).

consolidation of the packed powder by hot isostatic pressing (HIP).

In most cases a hot working step follows the HIP consolidation in order toefficiently reach a suitable size range. However the material can also beused in the as-HIPed condition for tool manufacture. Processing technologyin each of the manufacturing steps has a strong influence on the qualityof the final compact. The new PM plant in the Böhler-Uddeholm groupincorporates latest technologies in the field bringing the PM TS and HSSproducts to a new quality level.

POWDER MANUFACTURE

MELTING AND TUNDISH TECHNOLOGY

The main source creating inclusions in atomized powder is furnace slagor top slag in the atomization tundish. A generic feature of gas atomizationis the low metal flow rate, typically 20 - 40 kg/min. In order tomaintain


the required temperature in the tundish over the long atomization time thetundish had in the early days to be refilled frequently with superheated meltfrom the furnace. This brought entrapped furnace slag and tundish top slaginto the metal stream through the atomization nozzle.

An effective solution to this problem has been the introduction of largetundish technology in atomization units which started in the early 90’s. Herethe whole melt is transferred into the tundish prior to atomization and fre-quent violent stirring over the atomization nozzle during atomization canbe avoided. The large tundish must however be combined with asuitableheating system that can provide the energy necessary to maintain a sta-ble temperature over the long atomization time. The ElectroSlag Heating(ESH) method has proven to be ideal for this purpose. ESH operates with twographite electrodes generating Ohmic heat in a conductive slag on the metalsurface. The process offers effective protection to the melt against oxidationby the atmosphere. ESH further offers the great advantage ofcontrollingheat input and of stirring the melt independently as opposedto an inductionheating system. In the ESH process the melt is treated for a minimum of 30minutes in the tundish before start of atomization. During this time inclu-sions originating from the furnace slag are separated and the melt conditions(temperature, cleanliness) stabilized so that clean powder can be producedstraight from the start of atomization. The ESH process is basically notlimited by the alloy composition. As an example highly sulphurized grades(S∼ 0, 25%) are being produced using this technology with some processmodifications.The ESH process requires a gentle and controlled stirring ofthe melt so that heat generated in the top slag is evenly distributed in thewhole tundish and so that inclusions can be separated at the slag-metal in-terface. In the Böhler-Uddeholm plant the 8-ton tundish is equipped withan electromagnetic stirrer (EMS) which offers very good possibilities forprocess control as compared to gas purging, being the commercially usedalternative stirring method.

ATOMIZATION AND ENCAPSULATION

The demand for a high quality HSS powder with minimum contaminationcan best be fulfilled via gas atomisation with nitrogen gas. In order toachieve the highest possible uniformity in the capsule after encapsulationit is favourable to have a powder with smallest possible particle size since


large particles tend to segregate more than fine. This effectis illustrated inFig. 3.

In the Böhler-Uddeholm PM plant the atomisation process delivers a finepowder with average particle size around 60 micron (powder BUPT below)which is about half the particle size of traditional PM processes. The result-ing structure is thus very uniform in carbide size.

After atomisation the spherical powder is encapsulated in metal containersprior to consolidation by HIP. The handling of the powder until the capsuleis sealed is critical because of the risk of contamination byforeign particles(dust etc.). The handling should be kept to a minimum for thatreason. In theBöhler-Uddeholm plant the encapsulation follows directlyafter atomisationand is thus part of the atomisation process which practically eliminates anycontamination of the powder.

HIP CONSOLIDATION

Hot Isostatic Pressing of tool steel and high speed steel powder has provedto be the most suitable consolidation method to achieve fully dense anduniform PM blocks. The consolidation usually takes place atabout 100 MPaand at a temperature around 1150◦C. The time necessary to reach completepore elimination is depending upon the size of the capsule. Usually the timefor a full HIP cycle is many hours. The combination of the sizeof the press(i.e. weight of powder per HIP cycle) and the size of the capsule (i.e. holdingtime at temperature and pressure to reach full density) has to be optimisedfor best process economy in a large scale production operation.

HIP is not only influencing the cost of the PM compact but also the qualityto a large extent. It has turned out to be very important for the homogeneityof the capsule how the pressure and temperature is combined during theinitial phase of the cycle before holding conditions have been reached.

When a powder mass is heated in loose condition (no outside pressureacting on the can) and the powder starts to sinter at the rim, the volumeshrinkage caused by the neck formation between the powder particles resultin void channels in the powder mass leaving room for sulphur transport andsulphide growth (sulphur being a surface active element is present on theparticle surfaces). If improperly heated the sulphide growth can be excessiverendering the compacted capsule inferior properties. Thiseffect can be seenas a circular pattern in a sulphur etch of a cross section of a bar, Fig. 6. Inorder to avoid this effect the capsules must be heated under pressure whereby


the voids between the particles caused by the sintering shrinkage are keptclosed thus preventing any sulphide growth.

The 1st generation HIP units used external pressureless heating of thecapsules before they were hot loaded in the press. With the introduction ofthe cold loaded HIP presses in the mid 80s this way of HIP became state of theart. The early cold loaded presses however were equipped with molybdenumheating furnaces which required very long cycle times for HSS HIP sincethe load had to cool down to 200-300◦Cbefore the press could be opened.The latest development in this field is the introduction of the cold load-hotunload technology now used in the Böhler-Uddeholm plant. This combinethe high quality feature of heating under pressure (for bestuniformity of thecompact) with the cost efficient feature of quick unloading after completedconsolidation (for lowest HIP cost). The useful size of the pressure chamberis ∅ 1, 2 × 2, 4 m.

PRODUCT CHARACTERISTICS

The product properties of material made by the new manufacturing routein the Böhler- Uddeholm plant have been thoroughly investigated. Someresults are reviewed here.

CLEANLINESS

Non-metallic inclusions. Material is routinely checked for non-metallicinclusions according to DIN 50602, K0 and ASTM E45. The followinggraphs show the results from the evaluation of 51 samples made by the newPM process.

Another way of evaluating the level of non metallic inclusions in the steelis the so called MIC method (SIS standard SS 11 11 16) where thenumberof inclusions per surface area in a size class is plotted versus inclusion size.In a lin-log diagram the size distribution is represented approximately by astraight line. An example is given in Fig. 11 for the tool steel VANADIS 4.The number of inclusions larger than 22 micron is extraordinary few.

MECHANICAL PROPERTIES

Bend fracture strength (bfs testing using the 4-point bend testing methodon samples taken in the transverse direction show high values as illustratedin Table 1. Since bfs usually has a wide scatter in the resultseach value is a


mean of 10 individual samples. It is of interest to notice that the spread inthe values is lower than normally found in bend fracture testing of PM HSSand TS.

Table 1.

Steel grade Nominal composition Condition HRC Bend fracture strength, transverseaverage, MPa min-max, MPa

S 790 1,3C/4,2Cr/ As-HIP 64,7 4180 3872-4487Microclean 5Mo/6,4W/3VS 692 1,45C/4Cr/5Mo/ hot worked 97 % 64,0 4594 4043-4922Microclean 5,4W/3,7V/0,25SVanadis 4 1,5C/8Cr/ As-HIP 60,2 4720 4263-4983Superclean 1,5Mo/4V

The effect of hot working after HIP has been investigated. Itis oftenargued that hot reduction of the HIP-ed capsules results in better toughnesscompared to the as-HIP condition. Our results show that mechanical prop-erties in the as-HIP condition of 3rd generation PM HSS are similar to thoseof hot reduced material. Consequently further hot working after HIP is notnecessary for mechanical properties but needed in order to efficiently covera large size range of starting material for tool manufacture.

SUMMARY

The quality of the PM compact is the result of the combined efforts inevery processing step from raw material and melting to consolidation andfinal evaluation of consolidation result. Since the advent of PM HSS andTS manufacturing in the early 70s large improvements have been made inall steps in the manufacturing route. In the new process route in the Böhler-Uddeholm PM plant the latest technologies for PM HSS and TS have beencombined, that is

large tundish size

ESH

EMS

fine powder atomization

reduced and protected powder handling


cold load – hot unload HIP to full density.

The result of this optimised process route is a PM product with a very ho-mogeneous structure and very low level of non-metallic inclusions. For thetool user this means a more reliable and extended life of the PM tool.


Figure 3. Particle segregation after capsule filling of coarse and fine(BUPT) powder.


Figure 4. Powder particle size distributions in traditional and third generation PM process.

Figure 5. 1st generation PM HSS HIP cycle Hot loading – hot unloading.


Figure 6. Ring pattern with sulphur enrichment in HIP capsule after pressureless heatingbefore HIP.

Figure 7. 2nd generation HIP Cold loading – coldunloading.

Figure 8. 3rd generation HIPCold loading – hot unloading.


Figure 9. Cleanliness according to DIN 50602, K0.

Figure 10. Cleanliness according to ASTM E45.


Figure 11.

Figure 12.

Figure 13. Bend fracture strength vs. reduction ratio


Figure 14. The 3rd generation PM HSS and TS process.

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NEW OPTIMISED MANUFACTURING ROUTE