90-320Whitepaperonmachiningtoreduceriskofcrakingdiematerial

8/13/2019 90-320Whitepaperonmachiningtoreduceriskofcrakingdiematerial

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Introduction

Heat treatment of tool and die steels can be very complex and the heat treatment process must

be designed to maximize metallurgical properties of the alloy. This can subject the componentto a variety of thermal and transformation stresses. There is also a push in some industries toincrease quench rates to enhance material properties (i.e. H13 heat treated to NADCA 207 orGM DC9999-1). These more severe rates can result in higher stresses and higher risk.

To further complicate matters, most tool and die designs are very intricate, with unbalancedcross sections, thin webs, blind holes,and sharp corners. Tool and die steels by nature areextremely deep hardening.

Given the hardenability of alloys used, overall part geometries,and complexity of heattreatment, risks can be present for distortion or even cracking. These risks can be managedand reduced by understanding the factors involved and by taking steps to balance cross

sections, minimize stress, and reduce stress risers.

Understanding Sources of Stress

Stress is the main enemy in creating risk of distortion or even cracking. Stresses come mainlyfrom four categories:

Residual stress in the steel itself.

Stress created from manufacturing (i.e. machining, EDM, grinding, etc).

Thermal expansion and contraction from heating and cooling during heat treatment.

Transformation expansion and contraction. During heat treatment, one transformationcauses the steel to shrink and the other to grow. These are inherent metallurgical

transformation stresses. These stresses can be greatly influenced by the alloy type anduniformity of the material.

Residual Stress in Steel:

If you have ever machined a part and noticed that it was moving while you were machining it,you were experiencing the influences of residual stress. Even though the part is just sittingthere,it could be battling very high residual stress levels. You can think of it as certain fibers arein tension and others are in compression. Under these conditions, the tensile stresses willcompletely balance out the compressive stresses.

As you machine away fibers that are in either tension or compression, these stresses mustchange so they balance out again. Any time there is a change in stress,there has to be achange in strain (or simply there is a movement). The change in strain will follow the modulus ofelasticity for the particular alloy.

Once the die is heat treated, any residual stress will be relieved, and once again there can bestrain or potential movement in the die

Pre-hardened stock is known to have a very high level of residual stress.


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Manufacturing Stresses:

As material is removed during machining operations, you are literally exceeding the tensile

strength of the steel on a very small scale so you can remove the chip. This also creates adeformation zone surrounding the tool itself. Any time there is a deformation zone,stresses willbe created. The more severe the machining, the higher the stresses is. Once again, the mainrisk isthe relief of these stresses during heat treatment and movement of the die from this relief.

Other forms of machining,like grinding or EDM,can generate high temperature inputs if notperformed correctly. This can result in burned areas and heat affected zones. Under extremeconditions these processes may even create microcracks that could act as starting points forcrack propagation during heat treatment.

Special precautions should be observed for dies that will be wire EDMed after heat treatment.These precautions involve using higher tempering temperatures and the use of relief slots in

some instances. Please contact a Paulo Sales Engineer for additional information.

Stress concentration factors from machining are generally far more dangerous and risky thanresidual stresses created from machining. These will be addressed in much more detail later onin this paper.

Thermal Expansion and Contraction:

For most alloys, every inch of steel expands/contracts about 7 millionths forevery single Fchange. That may not sound like much, but a10 long bar of S7 tool steel will be a full 1/8longer when it is at its 1750F austenization temperature than it is at room temperature.

Growing 1/8 of an inch is not really the problem. The problem is that thin sections heat and coolfaster than thick sections and that the surface heats and cools faster than the core. Thesedifferencesin heating and cooling can lead to large stresses in the steel.

Forexample, lets look at a bar that is 10 long and 3 thick. As a theoretical example, uponheating if the surface is at 1500F and the core is at 1350F, then the surface will be a full0.010 longer than the core. It is easy to visualize how these stresses can multiply and howunbalanced cross sections can lead to yet higher stresses.

As a heat treater, we are very aware of these heating and cooling stresses. We usethermocouplesand monitor die temperatures and use pre-heats so surface and coretemperatures,as well as thinand thick sections,can equalize.

Metallurgical Transformation:

In the hardening of toolsteels there are two metallurgical transformations that occurone uponheating and one upon quenching. The transformation upon heating happens when the steelaustenitizes. When this occurs,the material actually shrinks. When the steel is quenched, themost significant change occurs. Upon quenching,the material hardens and forms martensite.Martensite is a significantly larger structure than the structure that was present before thetransformation.


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Upon heating, pre-heats are used to help minimize the impact of the transformation change toaustenite. The pre-heats help equalize the die temperatures so they transform more evenly.Remember,it is differences in size change that really relate to high levels of stress and risk.

The majority of risk is created during quenching. The steel must be quenched (at varying levelsdepending on the alloy), in order to harden out. Upon quenching,there will be differences intemperatures between the surface and core of the die. There will be differences in temperaturebetween thick and thin sections of the part. These differences result in stresses from thethermal contraction.

Upon quenching and once the steel cools enough to reach its transformation temperature,martensite begins to form. This temperature is different for every alloy, but in very generalterms,it is around 300-600F for most tool and die materials. This growth can be significantup to 4% larger than the structure it was formed from. This transformation is not instantaneous.Martensite only starts to form at its transformation temperature and will increase in

concentration as the steel continues to cool. Thus, as the part is quenched, you wind up withtwo gradients; temperature differences between the surface and the core,as well as martensitepercentage differences between the surface and the core.

The graph below shows a representation of the major size changes that occur during heattreatment. These are the sources that create risk of distortion or even cracking if not understoodand taken into account during the design and manufacturing stages.

3) Thermal expansion

upon heating

4) Thermal contraction

upon quenching

5) Expansion starts as

alloy starts to harden

2) First transformation

upon heating

1) Thermal expansion

upon heating

6) Fully

hardened.

Overall growthfrom original

Increasing Temperature

IncreasingS

ize


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As the tool is being quenched, the surface will harden first since it cools the fastest. This locksthe surface into place. As the core cools, it begins to transform and it then expands. You canthink of this as a balloon trying to expand from the inside.

This ballooning effect can result inhigh levels of distortion andmovement. Corners of cavities pullup. Knife edges bow. Under extremecircumstances,the part can alsocrack. This is why balanced andsymmetrical cross sections are soimportant.


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Tool steels as a group are very deep hardening. In other words, a high percentage of the overallcross section will harden out upon quenching. Some alloys are much more hardenable thanothers. Cold work tool steels,such as A2 and D2,will easily harden out half-foot thick cross

sections with high pressure vacuum furnace technology. Oil or water hardening steels,such asO1 or W1,require liquid quenches for much smaller cross sections. Remember, the moresevere or faster the quench, the greater the differencesin cooling rate and the resulting higherstress levels.

Larger blocks will experience much higher stresses simply due to the higher thermal andtransformation stresses. Given this fact, larger blocks will be at higher risk during heat treatmentand they should be under higher design scrutiny to minimize features that create stress risers.

Tool steel grades that contain a high level of carbide should also be under a higher designscrutiny to minimize stress risers. These grades, such as cold works, high speeds, or evensome stainless steels like 440C,obtain their high level of wear resistance from these carbides.

But high wear resistance comes at the price of lower ductility. Lower ductility alloys will be at agreater risk of potential cracking.

Premium grades of tool steel should used for higher risk designs. Alloys that are vacuum arcremelted will contain much more uniform chemistries and much lower contamination levels.

Assessing Risk

Features that act as stress risers should always be minimized whenever possible. The followingoutline presents a list of common features that should be included during risk assessmentduring die manufacture.

Machining Features on a Die that Can Increase the Risk of Distortion or Cracking during HeatTreatment

Rough Machiningo Deep tooling marks create stress risers.o Sharp Corners (inside)

Radii below 0.060 can be risky.Key ways/ slots/ grooves should be avoided.Counter sunk holes with sharp corners create stress risers.Blind holesare a high risk features.

o Transitions in cross section.

Major source of potential stress.Any change in cross section greater than 3 to 4 should be avoided.Thin webs, thin walls are problems.

Particularly risky if connecting changes in cross section


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o Water line placementCan be risky if too close to the surface.

Can be risky if too close to other die features (holes, changes in crosssection)Mismatch of cooling channels can leavesharp transitions on the internalsurfaces of the water lines. This can be checked by fishing a line throughthe cooling channels for match up.

o Sharp threads or gear roots create stress risers.

EDM before heat treatment (micro cracks in recast layer).o Recast layer should be removed.

Weld repairo Welding in general is a very risky procedure on tool and die alloys. It should be

completely avoided if at all possible.o It is recommended that the die builder notify the heat treater if any welding was

done to the die.o Preheat/post heat practice

Follow recommended practice from material manufacturerAt minimum,stress relieve prior to heat treatment

Welding creates very high localized stress levels. Stress relievingis strongly recommended to help relieve these localized stresses.

o Additional risk is present if welds are close to other features such as holes,corners, etc.

o Rough weld beads create higher stress risers. Beads should be smooth.

Thermocouple holeo In order to minimize risk, it is best to monitor the actual diewith a thermocouple.

If there is no thermocouple hole present, the heat treater must use a load blockthat may not completely represent the actual die temperature.

Thermocouple holes should be:

Surface 1/8 dia x 5/8 deep.

Core 1/8 dia. TC at center of heaviest mass or cross section.

Toughness(Charpy)specimens that are welded to the block can create stress risers.o Preheating, post-heating should be performed on the weldo Watch the location of welds on the specimen. Stresses can be generated at the

corners of the weld and toughness specimeno Do not use dissimilar weld filler


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Discussion and expansion of the out line

Rough MachiningRough machining can have two meanings here. One is the idea the die is machined to itsapproximate size of final dimensions. For the sake of this discussion we are not concern withthe rough size other that when the die is machined and the surface is left rough and has sharpcorners and/or small radii. It is the rough texture and sharp edges that can lead to anundesirable concentration in stress. The following are some examples:

Deep tooling marksHere in Figure 1A, an end mill cut too deep, essentially leaving a sharp inside corner. This wascleaned up to reduce the risk of cracking. In Figure 1B,a rough finish was left that has a ripple

appearance. When this type of feature is gross or coarse enoughit can act as a stressconcentrator, especially when it is directly associated with other features,such as a change incross section.

Figure 1.

A B


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Sharp Corners (inside)Sharp inside corners are of concern over anoutside corner. This is especially true for areaswith changes in cross section. It is recommendedto use a0.060 minimumradius when possible tohelp reduce an inside corner as a stress riser.

In Figure 2, a die insert is shown with a crack atan inside corner. This occurred during heattreatment but was attributed to the sharp radii atthe inside corner.

In Figure 3, a die with a large change in crosssectional area directly associated with a small radiusis shown. This has a great risk of cracking. The verysharp interface between these two sections willgenerate large stresses.

In contrast to the die above, the die in Figure 4 has a

very generous transition between changes in crosssection. This risk of cracking from the machiningfeatures is negligible here. It is important to do yourbestat making smooth transitions between large changes incross section.

Crack at a sharpinside corner.

Figure 2.

Figure 3.

Figure 4.


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Key way/SlotsKey ways and slots are a combination of inside corners and changes in cross section. Becauseof this,they can be a very risky area for generating cracks. Their location on the part withrespect to holes and changes in cross section also needs to be taken into consideration.

A slot with a sharp corner is shown in Figure 5.As can be seen,there is a crack at the sharpcorner.

Counter sunk holesFigure 6 has counter sunk holes which fall into a couple of categories. The photo on the rightshows a close-up of a hole. There arethree large risk factors present:(1) a sharp radius, (2) alarge transition in cross sectional thickness is present and,(3) it is a blind hole. This partcracked even though a precaution was taken by packing the hole with steel wool. Steel wool isone of the few tools on the heat treaters side to help reduce quench severity in a localized area.As can be seen, its not a guarantee, only an insurance policy to help reduce the risk.

crack

Figure 6.

Figure 5.

crack


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Blind holesBlind holes are holes that do not go through the whole cross section, and they are particularlyrisky. Risk increases if the bottom is left rough with sharp corners. Risk also increases if theyare close to other features, like changes in cross section, webs, etc.

Transitions in cross sectionA change in cross section greater than 3 to 4would be considered significant. The part shownin Figure 7 is an example of a significant changein cross section. The simple geometry andgenerous radius at the change in cross sectioncan reduce the risk of cracking. The issue withthis type of cross sectional change is for thedifference in heating and cooling. Upon heating,the small section will be at heat sooner andlonger than the larger section. The opposite istrue for cooling. Upon cooling,severe

metallurgical stresses can be set up by the factthat the small section will complete itsmetallurgical transformation before the largesection. This means the small section with behard and ridged while the large section is stillexpanding from the metallurgical transformationoccurring.

Figure 7.


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Thin websThe photo shown in Figure 8 provides an example of a thin web bridging a change in crosssection. Not only is the web thin, it also tapers down to a point. Note the crack in the thin web.

In Figure 9, the photo shows thereplacement die for the one above withthe web removed. This was successfullyprocessed. .

Figure 8.

crack

Figure 9.


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Water line placementWater cooling lines can also create stress risers and produce regions of risk. Risk is high if thecooling lines are close to the surface or to other features such as;changes in cross section,webs, etc. If the cooling lines are mismatched, they can leave sharp transitions on the internalsurfaces. These sharp transitions can be locations where cracks could start. Sharp transitionscan be checked by fishing a line through the cooling channels for match up.

EDM before Heat TreatmentWhen EDM is done before heat treatment,some caution should be exercised. Therecast layer can have micro-cracks, whichcould propagate during heat treatment.Figure 10 is a part that was EDM andcracked. Removal of the recast layer can bedone to reduce this risk.

Special precautions should be observed fordies that will be wire EDMed after heattreatment. These precautions involve usinghigher tempering temperatures and the useof relief slots in some instances. Please

contact a Paulo Sales Engineer for additionalinformation.

Weld repairWeld repair should be avoided when at all possible. There is a potential for cracking in a dieeven when proper procedures for welding have been followed. Welding is so risky the die couldcrack long before it is heat treated. The die builder should notify heat treater if any welding wasdone. It is best to follow recommended practices from the material manufacturer for welding. Astress relieve prior to heat treatment should be done as a minimum. However, for the best

protection, annealing of the die is recommended.

Figure 10.


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Rough or smooth weld bead

In Figure 11, photos show before and after a weld was cleaned up. The weld covered a largearea on one side of a die. The weld was extensive, rough, and of significant concern forpossible cracking during heat treatment. A stress relieve was done to the die to minimize therisk.

In Figure 12, two photos show a rough weld bead and the weld in close proximity to a bolt hole.

Both raise the risk of cracking. Even if a stress relieve or an anneal were done, it would berecommended that a weld this rough be machined smooth before heat treatment.

Figure 11.

Figure 12.


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Figure 13is an overview and close-up of a die with a weld at a change in cross section. Thetoe of the weld is right at the intersection of the two sections. Very risky, and as can be seen,

cracking was an issue. Full metallurgical analysis showed the crack started at the weld.

Minimizing Risk

If you want to heat treat a die, it simply must be exposed to temperature and metallurgicalstresses. The greatest way to reduce risk is to understand the sources of stress and to takesteps to minimize stress. Balance cross sections. Remove sharp edges. Blend and smoothsteps. Do not repair weld unless you simply must. Keep cooling lines away from surfaces andother features. Do not be afraid to leave extra stock in locations.

Work closely with your heat treater. Ask questions. Let the heat treater know when thesefeatures will be present. Let the heat treater know the final operating environment. The heattreatment cycle can be tweaked for insurance policies. Chances of success always increase as

communication increases.

Paulo Products Company

Joe PriceMetallurgical Engineer(314) 450-4349

[email protected]

Figure 13.
http://www.paulo.com/

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90-320Whitepaperonmachiningtoreduceriskofcrakingdiematerial

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