Dr. Joachim Boßlet / Michael Kreutz - TS Hungary · TUFFTRIDE®-QPQ-Process Salt bath...

TUFFTRIDE®-QPQ-PROCESSDr. Joachim Boßlet / Michael Kreutz

Technical Information

TUFFTRIDE®- QPQ - Process

Salt bath nitrocarburizing by the TUFFTRIDE process hasbeen applied in a wide range of industries throughout theworld for many decades. It is used to improve the wearresistance, the fatigue strength and – in particular whencombined with the oxidative cooling – the corrosionresistance of components made from steel, cast iron andsintered iron materials. In many cases the TUFFTRIDEprocess is used as an alternative to other surfaceengineering processes such as case hardening, galvanic(e.g. hard chrome plating), and other coating processes(plastic coating, painting, laser coating etc.), also plasmaor gas nitrocarburizing with equally good or improvedquality and greater economy.

Carrying out the process

Compared with other nitrocarburizingprocesses, the TUFFTRIDE Q, QPand QPQ processes are very easy tocarry out. As is usual when treatingcomponents in salt baths, the partsare first preheated to about 350°C inair. Nitrocarburizing takes place in aso-called TF 1 bath at 480 - 630°C,the standard temperature is usually580°C.

The salt melt mainly consists ofalkali cyanate and alkali carbonate.It is operated in a pot made fromspecial material, and the pot is fit-ted with an aeration device. Theactive constituent in the TF 1 bathis the alkali cyanate. During thenitrocarburizing process a reactiontakes place between the surface ofthe components and the alkali

cyanate, resulting in the formation of alkali carbonate. Byadding specific amounts of the non-toxic regeneratorREG 1, the nitriding active constituents are again produced in the salt melt and the activity of the TF 1 bathis kept within very strict tolerances (Fig. 1).

TUFFTRIDE® is a registered trademark of Durferrit GmbH.

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As the regeneration takes place without any change inthe volume of the nitrocarburizing bath, part of the meltdoes not have to be bailed out. Drag-out losses whichoccur when the parts are removed from the bath aresupplemented with TF 1 replenisher salt. Unlike gasnitriding or gas nitrocarburizing, the substances – TF 1and REG 1 - needed for the TUFFTRIDE QPQ process,do not contain constituents classified as toxic or harmfulto the environment.

A specially developed cooling bath (AB 1 bath) is usedfor carrying out the oxidative treatment after salt bathnitrocarburizing. During this treatment a black iron oxidelayer (magnetite) is produced on the surface of the treatedparts, which greatly enhances the corrosion resistance.The temperature of the cooling bath is 350-400°C. Apartfrom the oxidative effect, the bath has a positive influence on the dimensional stability of the cooled com-ponents.

Thereafter, the parts are cooled to room temperature andthen cleaned (TUFFTRIDE Q process).

If the surface of the components after nitrocarburizing isnot smooth enough for certain applications, dependingon the size and shape of the parts, various polishingmethods can be used to reduce the roughness(TUFFTRIDE QP process). Some proven methods are:

• Lapping with emery cloth grade 360 or finer;• Polishing or continuous microfinishing with special

plastic discs similar to centreless polishing, or on an automated lathe fixed between centre pieces or clamped in;

• Polishing in a vibrating drum. This method is primarilyused for small and thin parts;

• Blasting with glass beads size 40-70 µm in diameter. To prevent edges being excessively rounded off, or the thickness of the compound layer reduced, the pressure should not exceed 4 bar;

• Automated blasting with metal shot, if possible the diameter should be less than 1 mm.

Mechanical processing can, however, partly reduce thecorrosion resistance gained. For this reason, in manycases an oxidative post treatment in an AB 1 bath is carried out after polishing.

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This complete process sequence is shown in Fig. 2 andis in fact the TUFFTRIDE QPQ process. QPQ meansQuench Polish Quench and comprises TUFFTRIDEtreatment with oxidative cooling, mechanical processingand oxidative post treatment in a salt melt.

Composition andthickness of thenitride layer

Compound layer

During salt bath nitrocarburizing bythe TUFFTRIDE process a nitrocar-burized layer is formed consistingof the outer compound layer (ε -ironnitride) and the diffusion layer there-under. The formation, micro-structure and properties of thecompound layer are determined bythe base material.

The compound layer consists ofcompounds of iron, nitrogen, car-bon and oxygen. Due to itsmicrostructure, the compoundlayer does not possess metallicproperties. It is particularly resis-tant to wear, seizure and corro-sion, as well as being stablealmost to the temperature atwhich it was formed. Comparedwith plasma or gas nitrocarburiz-ing, compound layers with thehighest nitrogen content can beobtained by the TUFFTRIDEprocess. Layers with a high nitro-gen content give better protectionagainst wear, and in particular corrosion, than those with a lowcontent.

Depending on the material used,the compound layer will have aVickers hardness of about 800 to1500 HV. Fig. 3 shows a compari-son of the surface layers producedby various processes and theirhardness.

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In the metallographic analysis of salt bath nitrocarburizedcomponents, that part of the total layer known as thecompound layer is defined clearly from the diffusion layerthereunder as a slightly etched zone. During the diffusionof atomic nitrogen the compound layer is formed. Thegrowing level of nitrogen results in the limit of solubility inthe surface zone being exceeded, which causes thenitrides to precipitate and form a closed compound layer.

In addition to the treatment parameters (temperature,duration, bath composition), the levels of carbon andalloying elements in the materials to be treated influencethe thickness of layer obtainable. Although the growth of the layer is lower the higher the content of alloy, thehardness however increases to an equal extent.

The data shown in Fig. 4 weredetermined in a TF 1 bath at580°C. With the usual treatingdurations of 60-120 minutes, thecompound layer obtained was10-20 µm thick on most qualitiesof material.

Diffusion layer

The depth and hardness of thediffusion layer are largely deter-mined by the material. The higherthe alloying content in the steel,the lower the nitrogen penetrationdepth at equal treating duration.On the other hand, the hardnessincreases the higher the alloyingcontent.

In the case of unalloyed steels, the crystalline structure ofthe diffusion layer is influenced by the rate of cooling afternitrocarburizing. After rapid cooling in water, the diffusednitrogen remains in solution. If cooling is done slowly, orif a subsequent tempering is carried out, some of thenitrogen could precipitate into iron nitride needles in theouter region of the diffusion layer of unalloyed steels. Thisprecipitation improves the ductility of nitrocarburizedcomponents. Unlike unalloyed steels, part of the diffusion layer of high alloyed materials can be betteridentified metallographically from the core structure, dueto the improved etchability.

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But the actual nitrogen penetrationis also considerably deeper than thedarker etched area visible metallo-graphically.

Cooling does not influence the for-mation of the diffusion layer to anynoteworthy extent. Fig. 5 showsthe depth of nitration on variousmaterials in relation to the nitrocar-burizing duration.

Surface hardness and tensile strength

The surface hardness obtainableby TUFFTRIDE treatment is mainlyinfluenced by the composition ofthe material. The higher the content of nitride-forming alloyingelements (Cr, Mo, Al, V, Mn, Ti, W)the greater the surface hardness.Fig. 6 gives the average tensilestrength and surface hardness ofsalt bath nitrocarburized steels.

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Changes in component properties through the QPQ treatment

Corrosion resistance

To determine the corrosion resistance of samples andcomponents, a salt spray test (German Standard DIN50021) and a total immersion test (German StandardDIN 50905/part 4) are often carried out.

In the simple salt spray test theparts are subjected to a fine mist ofa 5% solution of sodium chloride at35°C. This test is referred to in theGerman Standard as SS.

Fig. 7 shows the results of a saltspray test conducted in accor-dance with DIN 50021 SS on hardchrome plated piston rods andTUFFTRIDE nitrocarburized onesmade from unalloyed steel C35.The piston rods were either hardchrome plated to a layer thicknessof 15-20 µm or salt bath nitrocar-burized for 90 minutes to obtain acompound layer 15-20 µm thick. Inthe case of the salt bath nitrocar-burized piston rods, different vari-ants such as nitrocarburizing plusoxidative cooling, with and withoutlapping, as well as the QPQ treat-ment were tested. After beingsprayed for 40 hours, the first cor-rosion spots occurred on thechrome plated piston rods. After180 hours the rods showed veryheavy corrosive attack over a largearea. All nitrocarburized pistonrods, however, were still free fromcorrosion after 40 hours and evenafter 180 hours the QPQ treatedpiston rods showed no signs ofrust.

Fig. 8 shows the corrosion resis-tance measured in a DIN 50021 SSsalt spray test of samples madefrom material C45 after each stageof treatment. Fig. 9 shows therespective surface roughness ofthe samples.

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In the ground condition, corrosionoccurred after only a short time.After 90 minutes salt bath nitro-carburizing followed by oxidationin the cooling bath the corrosionresistance was over 200 hours.Lapping does not change theresistance of the samples. Afteroxidative post treatment in thecooling bath (25 mins. at 370°C),figures of over 400 hours will beobtained.

The most stringent corrosion testunder DIN 50021 is the CASStest in which the test solutionadditionally contains acetic acidand copper chloride, and the tem-perature is raised to 50°C. Fig. 10shows the results obtained in acomparison between QPQ treatedpiston rods and hard chrome plated ones with layer thicknessesof 10-12 µm and 30-35 µm.

The test was performed by theMaterial Testing Institute inDarmstadt, Germany under thefollowing conditions:

Spray solution 5 % NaCl + 0.26 g CuCl2/l; pH 3.11-3.3; Temperature 50°C (1 test cycle = 1 hour).

After QPQ treatment, the corro-sion resistance is much betterthan after hard chrome plating.After 16 hours the QPQ treatedsamples merely showed corrosiveattack on about 10 % of the surface.

For the total immersion test (DIN50905/part 4) a solution of 3 %common salt and 1 % hydrogenperoxide (H2O2) is used as thecorrosive medium. Prior to beingdipped into the solution, the samples are degreased.

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Fig. 11 shows the results obtained on samples madefrom C45 treated by different surface engineeringprocesses after a total immersion test lasting 2 weeksand carried out in accordance with the Standard.

In the first horizontal column thereis the treatment and the averageweight loss per m2 and 24 hourson the QPQ sample.

With a weight loss of 0.34 g/m2,this sample is much better thanthe galvanic or chemically coatedones.

In the case of 12 µm hard chromeand even 45 µm double chromeplating, the weight loss wasaround 7 g/m2 and was therebymore than 20 times greater thanthat of the nitrocarburized sam-ples.

In the age hardened condition,the 20 µm nickel layer showed aweight loss of 2.9 g/m2. Only theTriplex layer containing 37 µmcopper, 45 µm nickel and 1.3 µmchrome is comparable with theTUFFTRIDE QPQ salt bath treat-ed sample.

Wear resistance and running properties

Due to the intermetallic composition of the compoundlayer, the friction and the tendency to weld with a metal-lic counter-partner are reduced. Excellent sliding andrunning properties, as well as greater wear resistance,are the well-known advantages of TUFFTRIDE treatedcomponents.

Wear tests and practical application repeatedly confirmthe superior wear resistance of salt bath nitrocarburizedparts over traditional or induction hardened or hardchrome plated surfaces. In very many cases, the wearresistance of the compound layer is improved still furtherby an oxidative post treatment. For example, compo-nents such as transmission shafts, plug gauges andhydraulic aggregates have a longer service life afterTUFFTRIDE treatment than after hard chrome plating.

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The question is often raised as to the wear resistance ofthe diffusion layer. Fig. 12 shows a comparison of thewear behaviour of rocker arms treated by two differentheat treatment processes. It shows the wear on the run-

ning surface of the rocker armwhich run against a salt bathnitrocarburized camshaft madefrom chilled cast iron. Althoughthe surface hardness of the casehardened rocker arm was slightlyreduced by nitrocarburizing, themuch improved wear resistancedue to the presence of the com-pound layer, to approximately 80hours running time is clearly visi-ble.

After 70-80 hours, the wear pro-file then runs parallel to that of thecase hardened only rocker arm,which is attributable to the pro-tection given by the diffusionlayer. A spontaneous increase inwear after the loss of the com-pound layer was not observed.

This test again showed veryimpressively that a high surfacehardness does not automaticallymean that the protectionagainst wear is also very high. It depends on the respectivewear mechanism involved as tohow a material or material part-nering is to be assessed. Nitro-carburized running partnershave proved themselves to bevery good under adhesive wearconditions in particular. Theirtendency to seize is muchlower than that of other surfacelayers.

Fig. 13 shows the results accord-ing to Nieman-Rettig of scuffingload limit tests on gears. Thesedata were established by apply-ing torque to the tooth flank and increasing it until seizureoccurred. Nitrocarburizing by theTUFFTRIDE process raised thescuffing load limit of the materialstested by 2-5 times.

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Another interesting factor in connection with the wearresistance and running properties is the friction coeffi-cient of the outer surface layer. The interfacial reactionswhich occur during sliding are not so much determinedby the absolute hardness of the running partner but bythe material partnering, their microstructural composi-tion, surface geometry and the lubricant used.

To determine the coefficient offriction, tests were carried out inour laboratory on the Amslermachine.

The tests were carried out withone disc running at 200 rpmagainst another disc which wasfixed. Both parts were treatedequally. To avoid adhesive wear, aload of 5-30 N was applied.Under greater loads the coeffi-cient of friction increased with theload but in the range of 5-30 N itremained constant.

Fig. 14 gives an overview of thefriction coefficient of different pair-ings under dry running condi-tions, and after being lubricatedwith oil, type SAE 30.

After hard chrome plating, case hardening, and nitrocar-burizing followed by water cooling or oxidative cooling inthe AB 1 cooling bath, the samples tested had a surfaceroughness of around 4 µm. Only the surfaces of theQPQ treated samples were reduced to a surface rough-ness of Rm = 1 µm by polishing.

Under dry running conditions, nitrocarburized sampleshave a much lower coefficient of friction than case hard-ened or hard chrome plated ones. Due to the oxidationof the compound layer, the coefficient of friction of thenitrocarburized samples increases.

In the lubricated condition, the hydrodynamic load sup-porting film has to be taken into account. With theexception of the QPQ treated samples, there is moresolid mass because of the surface roughness so that theresults presumably lie within the mixed friction range.Under these test conditions, of all variants the QPQ nitro-carburized samples had the lowest friction coefficient.

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The TUFFTRIDE treatment increases the rotating bend-ing fatigue strength and the rolling fatigue strength ofcomponents. These are mainly influenced by:

• the level of nitrogen in the compound and diffusionlayer,

• the thickness of the diffusion layer and

• the state of solution of the nitrogen on unalloyed steels.

Furthermore, the state of the microstructure and thestrength are to be taken into consideration. Whereaswith unalloyed steels the increase in fatigue strength isdetermined by the rate of cooling, with alloyed mate-rials, however, it has no mentionable effect. The in-crease in fatigue strength possible after 1-2 hours

TUFFTRIDE treatment is 100 % onparts made from unalloyed andlow alloyed steels.

In this connection we would like topoint out that hard chrome platingreduces the rotating bendingfatigue strength of the base mater-ial. A similar situation prevails withelectro galvanizing. Nitrocarbu-rizing, however, always increasesthe fatigue strength.

Fig. 15 shows the results of afatigue strength test conductedon notched samples made frommaterial C45N. QPQ treatmentincreased the fatigue strength bymore than 50 %. Hard chromeplating, however, reduced thefatigue strength by 20 %.

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Practical application of the TUFFTRIDE QPQ process

A variety of methods were used to test the resistance tocorrosion and wear, and the fatigue strength of compo-nents treated by the environment-friendly QPQ salt bathnitrocarburizing process. These demonstrated the superiority of this process over hard chrome plating,nickel plating and other nitrocarburizing processes.

Fig. 16 shows quick-fit connec-tions for joining hoses for liquidand gaseous media which used tobe manufactured from corrosion-resistant steels. By using the QPQsalt bath nitrocarburizing process itwas possible to change from theexpensive base material to anunalloyed case hardening steel. Inthe QPQ treated condition therequired corrosion and wear resis-tance is achieved without anyproblems.

Fig. 17 shows a section of a crank-shaft made from material 42CrMo4for high performance two-strokeengines. The QP treatment gavethe parts the required resistance towear and fatigue, and good run-ning properties.

The components were polishedautomatically in an overhead railshot blast machine with metalshot. Tests with gas or plasmanitrocarburized parts did not pro-duce the desired results.

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Fig. 18 shows gas springs and QPQ treated pistonrods. These parts are mainly built into tailgate flapsand engine bonnets of cars, as well as into the bag-gage flaps and cabin doors of aeroplanes.

It is reported that, compared with the hard chromeplating previously carried out, the corrosion and wearresistance is considerably improved. Furthermore, thecost of treating these parts was reduced by aboutone third.

Fig. 19 shows small parts such as fastenings for suit-cases and hairdryer grilles. These parts are treated inbulk and in very large quantities by the QPQ process.A nitrocarburizing duration of 60 minutes plus oxida-tive cooling, vibratory polishing and oxidative posttreatment are enough to obtain the required corrosionand wear resistance.

Representing the greatly diversified applications of theQPQ process are the typical components in Fig 20,which are nowadays treated in large quantities by thecombined salt bath nitrocarburizing process. Most ofthe parts involved, for example windscreen-wipershafts, v-belt tighteners and valves, are used in theautomotive industry.

The QPQ process is also used for components in theaircraft industry, in off-shore technology, in the con-struction of plant and machinery, in energy technology,in the food industry as well as in the manufacture oftextile machines, hydraulic aggregates and opticalequipment.

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TUFFTRIDE® plants and their economics

When choosing manufacturing processes,economics, quality and environment-friendli-ness are the most important criteria.Furthermore, the fact that they should be easyto carry out is of considerable importance.Compared with other nitrocarburizingprocesses, the TUFFTRIDE process is veryeasy to carry out. The treatment can be donein manually operated and fully automatedplants.

Modern, computer-controlled plants are veryversatile. Not only can they be adapted tomeet fluctuating production but also permitdifferent programs to run simultaneously.

The automated salt bath plant shown in Fig. 21is ideally suited for the TUFFTRIDE treatmentof mass produced parts in a production lineand for commercial heat treating which, dueto the different needs of the various cus-tomers, requires a heat treating plant to behighly adaptable.

Fig. 22 shows a schematic diagram of aTUFFTRIDE plant which, in accordance withmodern technology, is operated effluent-freeand which is equipped with an efficient extrac-tor system and exhaust air cleaning plant.With this plant technology the environmentaland work place regulations can be compliedwith in all industrial countries with no difficulty.

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A comparison of the economics ofthis process with other surfaceengineering processes also showssome favourable aspects. Costcomparisons made by varioususers showed that major savingscan be made. As an example,please see the cost comparison inFig. 23.

Low investment and energy costsalso had a very beneficial effect onthe economics of the process. Asaving of 37 % was possible withthe QPQ process over hardchrome plating.

Summary

In addition to the improvements in the properties such aswear protection, fatigue strength and sliding properties,the TUFFTRIDE treatment plus oxidative cooling and/orpost treatment produces a major increase in the corro-sion resistance. Results of tests and practical applica-tions show that the quality of the treated components isoften superior to that of electro galvanic layers and othernitrocarburizing processes. This opens a broad field ofapplication for the TUFFTRIDE process, which can oftenbe accompanied by expensive materials being replacedby lower-cost ones.

Due to the characteristics of the process, such as verygood repeatability of high quality results, easy use andhigh flexibility it is being used to an ever-increasing extentin the metal processing industry all over the world. Theprocess is very easy to carry out and does not requirecomplicated plant technology. The parts can be treatedin manually operated and computer-controlled plants.The plant itself operates effluent-free. The process ischaracterized by its extremely good environment-com-patibility. Therefore environmental regulations can easilybe complied with.

The TUFFTRIDE QPQ process is known in English-speaking and Asian countries under that name, inEurope and German-speaking countries as TENIFERQPQ and in the USA as MELONITE QPQ. TUFFTRIDE®,TENIFER® and MELONITE® are registered trademarks ofDurferrit GmbH.

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NOTES:

Details we provide on our products and appliances as well as our plants andprocess methods are based on comprehensive research work and operationalexperience. We provide such data either verbally or in writing to the best of ourknowledge and belief on the understanding that they do not present any warranty concerning the existence of specific characteristics in our products,but reserve the right to make technical modifications in the cours of productdevelopment.

This does not, however, relieve the user of his responsibility to check our dataand recommendations for his own application before using them. This alsoapplies - particularly for shipment abroad - with regard to the maintenace ofprotection rights of third parties and to applications and process methods notexpressly declared by ourselves in writing. In the event of loss our liability is limitedto replace goods and services on the same scale as is stipulated for qualitydefects in our General Conditions of Scale and Delivery.

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Durferrit GmbHIndustriestrasse 3D-68169 MannheimPhone +49 (0) 621 / 3 22 24-0Fax +49 (0) 621 / 3 22 24-809

www.durferrit.com E-Mail: [email protected]

Dr. Joachim Boßlet / Michael Kreutz - TS Hungary · TUFFTRIDE®-QPQ-Process Salt bath...

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