TUFFTRIDE -/QPQ -PROCESS - Metal Processing, Salt Bath ... · Salt bath nitrocarburizing by the TUFFTRIDE ... nitrogen could precipitate into iron nitride needles in the outer region

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

Technical Information

TUFFTRIDE®-/QPQ®- Process

Salt bath nitrocarburizing by the TUFFTRIDE® processhas been applied in a wide range of industries through-out the world for many decades. It is used to improve thewear resistance, the fatigue strength and – in particularwhen combined with the oxidative cooling – the corro-sion resistance of components made from steel, castiron and sintered iron materials. In many cases theTUFFTRIDE® process is used as an alternative to othersurface engineering processes such as case hardening,galvanic (e.g. hard chrome plating), and other coatingprocesses (plastic coating, painting, laser coating etc.),also plasma or gas nitrocarburizing with equally good orimproved quality and greater economy.

Carrying out the process

Compared with other nitrocarburizingprocesses, the TUFFTRIDE®, andQPQ® process are very easy to carryout. As is usual when treating compo-nents in salt baths, the parts are firstpreheated to about 350°C in air.Nitrocarburizing takes place in a socalled TF 1 bath at 480 - 630°C, thestandard 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® and QPQ® are registered trademarks ofDurferrit 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 products – TF 1and REG 1 - needed for the TUFFTRIDE® and QPQ®

process, do not contain constituents classified as toxicor harmful to 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 370-420°C. Apartfrom the oxidative effect, the salt melt has a positive influence on the dimensional stability of the cooled com-ponents.

Thereafter, the parts are cooled to room temperature andthen cleaned (TUFFTRIDE® 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. Someproven 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, may partly reducethe corrosion resistance gained. For this reason, in manycases an oxidative post treatment in an AB 1 bath is carried out after polishing.

2

This complete process sequence is shown in Fig. 2 andis in fact the QPQ® process. QPQ® comprisesTUFFTRIDE® treatment with oxidative cooling, mechani-cal processing and oxidative post treatment in a saltmelt.

Composition andthickness of thenitride layer

Compound layer

During salt bath nitrocarburizing bythe TUFFTRIDE® process a nitro-carburized layer is formed consist-ing of the outer compound layer (ε -iron nitride) and the diffusion layerthereunder. 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 TUFFTRIDE®

process. 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 main-ly influenced by the compositionof the 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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T = 580 °C C15

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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 (European Standard DINEN ISO 9227) and a total immersion test (GermanStandard DIN 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 EN ISO 9227 NSSon hard chrome plated piston rodsand TUFFTRIDE® nitrocarburizedones made from unalloyed steelC35. The piston rods were eitherhard chrome plated to a layerthickness of 15-20 µm or salt bathnitrocarburized for 90 minutes toobtain a compound layer 15-20µm thick. In the case of the saltbath nitrocarburized piston rods,different variants such as nitrocar-burizing plus oxidative cooling,with and without lapping, as wellas the QPQ® treatment were test-ed. After being sprayed for 40hours, the first corrosion spotsoccurred on the chrome platedpiston rods. After 180 hours therods showed very heavy corrosiveattack over a large area. All nitro-carburized piston rods, however,were still free from corrosion after40 hours and even after 180 hoursthe QPQ® treated piston rodsshowed no signs of rust.

Fig. 8 shows the corrosion resis-tance measured in a DIN EN ISO9227 NSS salt spray test of sam-ples made from material C45 aftereach stage of treatment. Fig. 9shows the respective surfaceroughness of the 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 EN ISO 9227 is theCASS test in which the testsolution additionally containsacetic acid and copper chloride,and the temperature is raised to50°C. Fig. 10 shows the re-sults obtained in a comparisonbetween QPQ® treated pistonrods and hard chrome platedones with layer thicknesses of10-12 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 theQPQ® treated 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 many cases, the wear resis-tance of the compound layer is improved still further byan oxidative post treatment. For example, componentssuch as transmission shafts, plug gauges and hydraulicaggregates have a longer service life after TUFFTRIDE®

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®

nitrocarburized samples had the lowest friction coeffi-cient.

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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 %on parts 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 TUFFTRIDE®

or QPQ® process. These demonstrated the superiority of this process over hard chromeplating, nickel plating and othernitrocarburizing 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 QPQ®

salt 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 TUFFTRIDE® treat-ment gave the parts the requiredresistance to wear and fatigue, andgood running 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.

12

Fig. 16

Fig. 17

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 quantitiesTUFFTRIDE® resp. Most of the parts involved, forexample windscreen-wiper shafts, v-belt tightenersand valves, are used in the automotive industry.

The QPQ® process is also used for components in theaircraft, motocycle and utility vehicle industry, in off-shore technology, in the construction of plant andmachinery, in energy technology, in the food industryas well as in the manufacture of textile machines,hydraulic aggregates and optical equipment.

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Fig. 18

Fig. 19

Fig. 20

TUFFTRIDE® plants and environmental uspects

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.

In addition an ecolocigal assess-ment of nitrocarburizing, pub-lished by the University of Bremenin 2001, resulted that from anecological point of view, salt bathnitrocarburizing is more favour-able than gas nitrocarburizing, ifthe study is considered objective-ly, the opinion often expressedthat salt bath technology harmsthe environment, and thereforedoes not conform to present-dayenvironment philosophy, cannotbe confirmed.

The results are summarized inFig. 23. To ensure a real compari-son, the nitrocarburizing of 1,390tons of crankshafts (135,000)made from steel 42CrMo4V inplants of a suitable size (salt bath

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Effluent free salt bath plant

Fresh Water

Waste WaterTank

Wet ScrubberEvaporator

Exhaust

Cascade

TUFFTRIDE TUFFTRIDE AB 1

Preheating

10 % 1.0 % 0.1 %

Cold WaterRinse

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furnace 0.75 m3, gas furnace3.6 m3) was analyzed. All thesources of energy, substances andmaterials involved in the thermo-chemical processes, going back totheir origins (production of energy,manufacture of materials required),were included and related to thenumber of parts treated. Theanalysis was followed by anassessment. “Detriment points”were given for the use of materials,substances and energy, also forwaste gases, effluents, and solidwastes. For better understandingthe environmental impact of 100cars driving 15,000 km each isalso given.

Summary

In addition to the improvements in the properties such aswear protection, fatigue strength and sliding properties,the TUFFTRIDE® treatment plus oxidative cooling orQPQ® produces a major increase in the corrosion resis-tance. Results of tests and practical applications showthat the quality of the treated components is often supe-rior to that of electro galvanic layers and other nitrocar-burizing processes. This opens a broad field of applica-tion for the TUFFTRIDE® or QPQ® process, which canoften be accompanied by expensive materials beingreplaced by lower-cost ones.

Due to the characteristics of the process, such as

• Excellent repeatability of high quality results• Shortest treatment times• Low disturtion• High flexibility

It is being used to an ever-increasing extent in the metalprocessing industry all over the world. The process isvery easy to carry out and does not require complicatedplant technology. The parts can be treated in manuallyoperated and computer-controlled plants. The plantitself operates effluent-free. The process is characterizedby its extremely good environment-compatibility.Therefore environmental regulations can easily be com-plied with.

The TUFFTRIDE® process is known in English-speakingand Asian countries under that name, in Europe andGerman-speaking countries as TENIFER® and in theUSA as MELONITE®. TUFFTRIDE®, QPQ®, TENIFER®

and MELONITE® are registered trademarks of DurferritGmbH.

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Evaluation containscontamination through::

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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.

05/0

720

00gr

iessDurferrit GmbH

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

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

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TUFFTRIDE -/QPQ -PROCESS - Metal Processing, Salt Bath ... · Salt bath nitrocarburizing by the TUFFTRIDE ... nitrogen could precipitate into iron nitride needles in the outer region