Oxidation Protection for Metals and Alloys The best protective and reaction gas for every application Under the brand name Neutrotherm ® , the Messer Group is a distributor of inert gases low in hydrogen, with which e.g. iron, copper and aluminium alloys can be protected against oxidation during heat treatment. Hydrogen-rich gas mixtures, which have proven themselves as a protective and reaction gas in the case of chrome alloys for instance, are distributed under the Messer brand name Hydrotherm ® . Hydrogen brings brightness and cleanliness. Technical press
Transcript
Oxidation Protection for Metals and Alloys
The best protectiveand reaction gas forevery application
Under the brand name Neutrotherm®,the Messer Group is a distributor of inertgases low in hydrogen, with which e.g. iron, copper and aluminium alloys can beprotected against oxidation during heattreatment. Hydrogen-rich gas mixtures,which have proven themselves as aprotective and reaction gas in the caseof chrome alloys for instance, aredistributed under the Messer brandname Hydrotherm®.
Hydrogen brings brightness and cleanliness.
Technical press
It is the nature of things: if metal is heated until itglows, it reacts with oxygen and oxidises. Initial-ly, only the unwelcome oxide layer is apparent;the drop in quality of the material usually onlybecomes clear at a later point in time. In order toprevent oxygen from the air being carried into thefurnace and reaching the batch material, themetallurgic heat treatment, i.e. the phase inwhich the required properties are practically burntinto the metal, takes place under an atmosphereof protective gas. In some cases, it is sufficientto use nitrogen (N2) as the inert gas to preventoxidation of the metal surface. However, de-pending on the alloy and the properties required,a reactive gas is additionally needed which isthen dosed into the furnace - gas which is dilutedwith nitrogen or, depending on the application,also with argon.
Perfect protection against oxidation thanks tohydrogen (H2)In the field of metallurgy, hydrogen has provenitself to be an ideal reducing agent. It is used,
to anneal stainless steel without oxidation,•for the re-crystallisation of heat-resistantspecial steels,
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for cleaning surfaces of metal by removingcarbon residues,
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for annealing non-ferrous alloys such as copper,bronze, nickel silver or brass,
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for increasing the cooling performance ofcontinuous annealing furnaces (in conjunctionwith liquid nitrogen),
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in high convection bell-type furnaces,•for sintering moulded components of iron and(non-ferrous) powders,
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for the reduction of metal oxides, such astungsten oxide, as well as
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for the blank annealing of hardened steels afterhardening.
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The Messer Group provides its customers withN2/H2 mixtures in two fundamentally differentconcentrations for every application requirement:
Collected together under the trade nameNeutrotherm® are "low hydrogen“ mixtureswith up to four percent by volume of hydrogen,which are suitable primarily as protective gasesin the heat treatment of iron, copper oraluminium alloys. The mixtures can be usedsafely at temperatures below 750 °C; they arenot explosive at room temperature either.
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The trade name Hydrotherm® stands for gasmixtures which can contain up to 100 percenthydrogen and are suitable as protective andreaction gases for chrome-rich alloys (heattreatment up to 1150 °C), and for avoiding anunwelcome oxide layer on the surface of thematerial, also at extremely high temperatures.
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Although the methods of heat treatment aresimilar in many respects, they cannot be reducedto a common denominator. On a case by casebasis, specifics have to be taken into accountwhich, for instance, make it necessary to modifythe composition of the protective and reactiongas mixture. Of general importance is theprinciple that, in metallurgy, a multitude ofapplications exist which depend on the use ofhydrogen - with or without reduction of oxides.
Examples of Applicationfrom Industrial UseAnnealing chromium-rich and scale-resistantsteelsAfter cold rolling, rust-resistant steel strip 1.4301(X5CrNi18 10) is continuously treated in a brightannealing furnace until re-crystallization; aftercooling to room temperature, it should becompletely bright when leaving the plant. Thesmallest change in the prevailing atmosphere inthe furnace could however lead to the initiation ofthe chromium-oxidation process. A dew point ofaround minus 25 °C, corresponding to 0.063percent by volume of water (H2O), and a furnacetemperature of 1050 °C, causes an alloy of 18percent by weight chromium to oxidise. Theequation of this reaction is:
2 Cr + 3 H2O ↔ Cr2O3 + 3 H2 (1)
The resulting oxide layer can vary in colour,depending on the temperature and time spent inthe furnace (see Fig. 1). With a shift from yellow-red to blue-green, the thickness of the layerincreases; the higher the temperature and thelonger the time spent in the furnace, the thickerthe oxide layer.
Figure 1: Kinetics of the formation of chromium oxide of an alloy with20% chromium content.
Coat thickness in micrometers
Blue coating
Yellow coating
Transparent coating
Temperature in K
2Cr + 3H2O = Cr2O3 + 3H2
The diagram shows the thickness of the oxidelayer to be expected at different temperaturesand oxidation times. Depending on the thicknessof the oxide layer, various annealing colours form.Under 0.03 µm, which corresponds to 300Ångström (1Å = 1*10-10 m), the very lightly-oxidised surfaces still appear to have a metallicbrightness.
Discolouration - the oxidation of the surface ofthe metal - can be prevented by using purehydrogen and a dew point of -50 °C and below,which equates to 0.004 percent by volume ofwater in the atmosphere of the hot furnace.Consequently, not only is the steady state of thewater content in the furnace atmosphereresponsible for the absolute "brightness" of thesurface, but also the conditions under which thesteel cools down. In order to prevent theformation of chromium oxide during the entireduration of heat treatment, the humidity in thefurnace should at all times be well below thestability line. This will prevent an oxide layer fromforming in the cooling phase, i.e. when tem-peratures decrease. If steels containing manga-nese, niobium and chromium as alloy elementsare to be protected from oxidation, then thesteady state dew point must be at -70 °C orbelow. In this extremely dry atmosphere, theformation of a grey or matt oxide layer on thesurface of the metal will be prevented. In orderto achieve this, the use of very dry technicalgases is necessary.
Figure 2 helps to illustrate to what extent themoisture content must be reduced in order toprevent oxide forming in the balance phase andduring the cooling of the alloy. The stabilityincreases in the sequence manganese, niobium,chromium, titanium, silicon and aluminium, theprevention of the formation of oxide becomesmore difficult. The oxidation of titanium, siliconand aluminium can generally not be prevented.
Sintering of metal powdersPressed parts of metal powders are sintered incontinuously operating furnaces. The pressedgreen sinter become stable and compact,provided that the atmospheric oxygen in thepores combines with hydrogen during theheating and sintering phase and the waterresulting from this is removed. If the water
is removed quickly, a selective oxidation of thealloy elements such as carbon, chromium andmanganese can be prevented.
Production of bi-metallic strips: Special bronzepowder is applied to the steel strip, then thepiece is sintered in the conveyor furnace, bothlayers being tightly bonded to each other.Figure 2: Oxidation limits of various steel alloys, depending on partial
pressure of oxygen, dew point and temperature.
Oxygen pressure in -log (bar)
Figure 4: Sinthering in conveyor furnace
Dew point
Temperature in K
Figure 3: Sintered pressed parts
- 70 °C- 60 °C- 50 °C- 40 °C- 30 °C
2 %AI/AI2O3
2 %Si/SiO2
1 %Ti/TiO
25 %Cr/Cr2O3
1 %Nb/NbO2
2 %Mn/MnO
Thanks to the use of very dry hydrogen and purenitrogen as diluting gas, the distribution of thealloy phases produced in the course of theproduction of powder does not change duringsintering. The phases remain intact and wellmixed. The prevailing moisture content in thevarious areas of the sintering furnace iscontinuously measured in order to calculate theminimum concentration of hydrogen necessary.
Annealing of copper and copper alloysPure copper is a very noble metal which formsoxide only with oxygen. At temperatures of up toaround 350 °C, a reddish copper oxide develops(Cu2O), at higher temperatures black CuO.
4 Cu + O2 ↔ 2 Cu2O + O2 ↔ 4 CuO (2)
Pure copper The soft annealing of copper can easily be carriedout using nitrogen with a small amount of hydro-gen added. Usually 2 to 4 percent by volume ofH2 is sufficient to convert the atmosphericoxygen which enters the furnace into water, i.e.before copper oxide forms. It is important toensure that when cooling the charge to below350 °C, the metal is not oxidised by atmosphericoxygen.
Nickel silverThe soft annealing of nickel silver (copper with20 - 30 percent by weight nickel) requires ahigher hydrogen concentration in order to preventthe selective oxidation of the nickel, which is notas noble as copper. A practical instance demon-strated that atmospheric oxygen entering thecontinuously operating pusher furnace formsnickel oxide on the material being annealed. Afterthe holding temperature had been reached, theoxide was reduced again by the hydrogen whichwas present, leading to a kind of "mirror" beingformed on the alloy. The demixing of the alloywould substantially reduce the corrosion resis-tance of heat exchanger pipes in subsequent use.
Brass and bronzeBrass is often annealed in air, even though a kindof passive zinc oxide layer forms on the surfaceof the metal, which means that further oxide canonly form very slowly. However, in some cases itmay be necessary to prevent the formation ofthis hard oxide layer. The optimal concentrationof hydrogen for the piece to be annealed iscalculated empirically, with the recrystallisationtemperature and the required properties of thealloy being taken into account.Bronzes must always be annealed in a controlledatmosphere. They do not passivate and in con-tact with air would oxidise to a high degree, alsoleading to the formation of tin oxide and copperoxide.In the processing of brass and bronze, gasmixtures with a hydrogen content of 20% areused. Clearly visible: with increasing hydrogencontent, the surfaces of the metal become lighterand more brilliant.
Figure 6: Copper tubes annealed in a hood furnace
Figure 5: Sintering of bronze powder on steel strip under controlled nitrogen/hydrogen protective gas
Hydrogen for cleaning surfacesHydrogen is suitable not only as a means ofreduction or in order to improve thermal char-acteristics, but also for cleaning surfaces. In thecourse of metal processing, extrusion or rollinglubricants often remain on the material. This is avery unfavourable situation in the sense that theresidues can burn into the surface during heattreatment. These unwanted residues can beremoved in the course of rinsing cycles usingnitrogen and hydrogen, carried out while thematerial is in the furnace. In this instance, adegree of purity is achieved which meets therequirements of e.g. the automobile industry ormedical engineering.
Reduce costs thanks to tailor-made gasmixturesEvery application requires its own adequateprotection and reactive gas. The choice ofprotective and reaction gas used (nitrogen,hydrogen, argon or mixtures) depends on thespecific requirements. The gases and gas
Gas Density [kg/m³] Thermal conductivity Specific heat capacity [kJ/kgK]
Table 1: Physical properties of gases at 25 °C and 1 bar absolute
mixtures required are put together individuallyand the quantities necessary determined on siteby trials. Only in this way can production qualitybe increased and operating costs reduced.The re-crystallisation annealing of scale-resistantsteels is still achieved today in individual cases byusing ammonia cracked gas. For this purpose,ammonia is cracked thermally in heated gen-erators. One cubic metre of ammonia producestwo cubic metres of cracked gas, which consistsof three parts hydrogen and one part nitrogen(see reaction 3).
2 NH3 ↔ 3 H2 + N2 (3)
The quality of the protective gas produced isdetermined primarily by two criteria: 1. the purityof the ammonia used and 2. as far as possible, afully completed cracking process. Even smalltraces of residual ammonia give a matt andgreyish appearance to the surface of the treatedsteel. However, this is avoided by the use of high-purity hydrogen and nitrogen. The "new processgas" used in heat treatment consists of technicalgases and features dew points of -70 °C andlower.As has been shown in practice, the new opti-mised gas mixture needs less hydrogen toachieve annealing results comparable to thoseachieved using ammonia cracked gas. In con-tinuously operating furnaces such as pusher orconveyor furnaces, the specific gas costs can bereduced noticeably as a result.A further advantage of the high purity of hydro-gen and nitrogen: The reforming time of a fur-nace is shortened after shutdown and annealingoperations can be started again without delay.
Use the properties of hydrogen for theprocessIn comparison to all other gases, hydrogen hasnoticeably higher thermal conductivity andspecific heat capacity. Nitrogen/hydrogenmixtures possess equally favourable thermalproperties. Among other uses, these mixturesare deployed in the annealing of strip coils or wirecoils in bell-type furnaces with high-convectiontechnology.
Figure 7: Coils on the floor of a bell-type furnace with high-convection technology
Hydrogen has 7 times more thermal conductivitythan nitrogen and 13 times more specific heatcapacity.
Consequently, hydrogen accelerates the transferof thermal energy to or from the charge beingannealed. At the same time, H2 ensures that theentire charge is heated through more evenly. Dueto the better temperature distribution within thecharge being annealed, the heating up andcooling down times for a load can be shortenedby up to 20 percent, so the output of a furnacecan be considerably increased. Purification systems for very high purity
gasesMesser offers its customers with a range of gas-purifying systems which allow gases of thehighest purity to be produced. In this way, res-idues of water and oxygen can be easily andsafely removed.
Figure 8: thermal conductivity of hydrogen and nitrogen
Thermal conductivity in W/mK
Thermal conductivity ofhydrogen is 7 times higherthan nitrogen.
Figure 10: Nitrogen/hydrogen-mixer for gas mixtures
Correct gas mixtures for first class results•Save costs by optimal hydrogen content•Reduced gas requirements for each application•Constant availability of hydrogen and nitrogen•Tried-and-tested secure storage•Low investment costs•
Advantages of hydrogen/nitrogen at a glance:
Higher purity of the technical gases•Measuring and concentration as needed•Consistent quality due to reproducible mixtures•Nitrogen as a safe purging gas•Less protective gas used•Better quality of finish•Increase in productivity possible•Flexibility when changing the annealing task•Increase in the range of products•
Hydrogen in the field ofmetallurgy A question of the right relationshipHydrogen (H2) in its purest form is the mostcommon element in the universe. Down hereon Earth, however, one seldom comes across iton its own but mostly in heterogeneous com-pounds, primarily with oxygen with which itforms the very stable compound water. This facthas led to it becoming the most important re-ducing agent in metallurgy. During the annealingof alloys, hydrogen is always in demand when aperfect surface is called for.
A little insight into the chemistry of hydrogen inthe heat treatment of alloys: H2 has a high affinityto oxygen; both react very easily leading to theformation of water. No solids are deposited onthe metal alloy. Whether or not an oxide formson the metal surface during annealing dependsessentially on the chemical composition of thealloy and on the effective atmosphere.Show me your oxide and I'll tell you what kind ofmetal you are: The stability of the oxide is char-acteristic for each metal. Whether it oxidises inthe furnace or its oxide is reduced by equation (4),
MeO + H2 ↔ Me + H2O (4)
depends primarily on which direction the reactionruns or on the prevailing concentration of waterand hydrogen as well as the respective tem-perature. The purity of the hydrogen used andthe condition of the furnace also affect thereaction procedure.Whether an existing furnace can be convertedfrom the previously used gas mixtures (such asExogas, monogas or ammonia cracked gas) tohydrogen and nitrogen depends mainly on thetechnical condition of the plant and also on thematerials to be annealed as well as the qualityrequired. Which concentration of hydrogen isnecessary for reliable and reproducible operationof the plant must be tested by trials on site ineach case. It is important that no oxygen,whether in compound or unbonded form, getsinto the furnace or the cooling area. In addition,the proportion of water to hydrogen createdshould be kept as low as possible in order toprevent an oxidation of the charge.The Messer Group not only has the necessaryknow-how when it comes to gases, the companyalso has experience in technical applicationswhich has been gained over many years in theimplementation of furnace concepts for the oxide-free annealing of metal alloys.
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