Sticking problems during heat treatment in vacuum furnaces

 

A forum discussion, that took place some time back in which I participated, brought me to the idea to write an article about a phenomenon in vacuum heat treatment, called Sticking. Obviously an issue not always understood or not taken care of to avoid this by heat treating operators working on vacuum furnaces.  Physical processes like sublimation, evaporation and solid-state diffusion contribute or are causing sticking problems, in fact parts contacting other parts or fixtures may stick together during vacuum operations. Another aspect not always considered during operating a vacuum furnace is the fact that friction coefficients in vacuum and at high temperature are higher than in air and ambient temperature, which may contribute to deformation. The first subject was raised by a heat treater who had problems with sticking problems when hardening high speed steel M4.

 It concerned running rolls, up to 15 kg stacked on top of each other. The hardening temperature was 1190 ⁰ C in a vacuum furnace at a partial pressure of 1.2mbar. The question was whether anybody experienced this issue or has any advice on how to prevent the sticking. 45 years after introduction of vacuum furnaces in our heat treatment branch, I considered it remarkable that this issue in vacuum heat-treating operations might still be a problem. As a matter of fact it is a basic physical issue of solid state diffusion that occurs at elevated temperature when contacting clean parts are under a certain load and low vacuum pressure, atoms of one part diffuse into the surface of a contacting part, as such creating a bonding between both parts. When burrs or other irregularities are present on the contacting surfaces, like metal dust or small metal particles are slept in, having a high vapour pressure, this may cause sticking as well. According to table 1 at a hardening temperature of around 1200⁰C, if the partial pressure in the furnace is less than 1.3 x 10¯²mbar chromium will vaporize. Although the pressure chosen of 1.2mbar is not wrong, however obviously too low in the range to avoid sticking. It is often better to select a much higher pressure to reduce evaporation and sticking, also taking into account the tolerance thermocouples and pressure gauges may have. A partial pressure in the range of 5- 7 mbar or even higher, taking into account the weight of the rolls of 15 kg stacked on each other, it would be more reliable to avoid sticking. In the mentioned forum discussion, about 20 comments on this issue were brought in. To avoid metal/metal contact the majority of the forum members suggested to use magnesium oxide, brazing stop-off, alumina paint, boron nitride spray, Fiberfrax, binder free ceramic-and regular paper, mesh made of Kanthal and so on. Some of them work well others require cleaning after vacuum heat treatment or disintegrate. Instead of the risk of contaminating the pump oil or condensation of vaporized metal elements e.g. chromium on cold places in the vacuum furnace, I preferred to suggest to raise the partial pressure, at the same time to put small ceramic bars between the rolls. The intention was not only solving the sticking problem but also to improve more uniform cooling. By putting ceramic bars or plates between the rolls and between the bottom roll and the grid or basket as such creating separation of the metallic surfaces as well as creating space between the rolls improving more uniform and faster quenching the outside as well inside diameter.

An additional advantage of separating component surfaces by ceramic is lowering the friction coefficient, which in vacuum and at high temperature can be 2-3 times higher than in air and ambient temperature. An important issue because the general opinion is that vacuum hardening renders the least deformation, which is not always true. Why?  Next time we will discuss this. 

Table 1.               Temperature (⁰C) at which specific vapor pressure (mbar) exists.    

Dokument11 page 001

Hans Vetrop, May 2016


 

To find the way in the nomenclature  jungle of nitrogen diffusion 

                                                             

More than once I was asked what I was doing for a living and when I answered “heat treatment”, the person thought I was working on central heating systems in homes and buildings.

No one hit on the idea of e.g. steel and other metals and alloys to make them more durable as a tool or a construction part in machinery by heat treatment.

When asked how they thought about making products by tools or car components like gears in gear- boxes to withstand a 100.000 miles and more without making these wear resistant  it was understood that without heat treatment we might still live in the early Middle Ages.

This introduction shows how many men and women in the street are aware of what heat treatment brought to them on comfort and allowing them to live a comfortable life.

Although according to archeologists evidence of steel hardening started 3000 years ago it lasted till the Middle Ages that alchemists described processes by which carburizing and nitriding have taken place obviously. Carried out by blacksmiths who annealed hot forgings in bird faeces and quenching knives and swords in boys urine, not knowing why, probably realized the first nitriding effect.

It lasted till the early 1900s when Machlet and Fry introduced the gasnitriding and use of ammonia  and Flow published the two-stage process reducing the compound zone.

In 1932 Berghaus and Wehnheldt developed the plasma-or ion nitriding process

Saltbath processes were developed by ICI and later acquired by Degussa introducing  Tenifer or Tufftride nitrocarburizing in the 50s.

These three base processes led to a large number of derivative processes developed later on  during which over the last decennia the controle of nitriding processes have been optimized.

The large variety of processes and trade names created by commercial heat treaters and furnace builders however may create confusion and misunderstanding.

Why?

World trading and global sourcing of parts and products can create purchase specification problems.

In the past consultation between components- or service supplier and buyer, when near-by, national or international  opposite to intercontinental was more extensive.

This gearing to one another often was not documented and more an unaware client service. This advising may be lacking in many occasions nowadays in particular when trading globally.

The consequence is that false expectations of norms arise as the requirements of the user may not be described in norms, e.g. steel parts to be surface hardened

The purchase order often contains only the name of a heat treatment e.g. nitriding or nitro- carburizing and a reference to a norm and price & delivery conditions. The required profile of the properties for supplying is often not clear and established. Manufacturing, processing, delivery according to a certain norm guarantees normally a standard quality.

The requirements regarding the properties of the heat treatment are sometimes more comprehensive than the standard quality.

Specifying Requirements

There are standard norm systems globally, covering all kind of details on the processing of nitriding and nitrocarburizing  however many individual suppliers prefer to keep their processing confidential.

Therefore besides referring to a norm it is essential to specify certain or better specific requirements on max. temperature, required structure, surface hardness, effective nitriding depth and so on.

Confusion

Since some decades tremendous developments on diffusion processes  like nitriding, nitro- carburizing and surface engineering (Coatings) have seen daylight and implemented into the heat

treatment world. Almost all furnace suppliers and commercial heat treatment shops with or without own input, trademarked their processes. The consequence is that people, not involved in heat treatment, are not aware what is behind the fancy proprierty names.

Global sourcing has increased the confusion buyers and design engineers experience.

It is not sufficient to refer only to existing norms, as they may not reflect what is needed.

Culture and  language differences may increase mutual misunderstanding.

Temperature ranges of Nitriding / Nitrocarburizing ( 2)

Low Temperature Nitriding / Nitrocarburizing for stainless steels                      Max    N 450 ⁰C,  C 550⁰ C

Nitriding: Class 1/ Class 2 (Floe process)                                                                                   500-565⁰ C

High/Low pressure

Oxynitriding, Sulfonitriding, Oxysulfonitriding                                                                            510- 580⁰ C

Ferritic nitrocarburizing                                                                                                              550- 585⁰ C

Austenitic nitrocarburizing                                                                                                          595-720⁰ C

High temperature nitriding                                                                                                         700- 800⁰ C

Solution nitriding of stainless steels                                                                                        1050-1150⁰ C

If the core hardness is important, tempering of pre heat treatment should be about  30⁰ C higher than the  nitriding/nitrocarburizing temperature

For Classic nitriding single and two-stage the norms are clear also for no experts.

SAE AMS 2759/6 specifies the nitriding of low- and higher alloy steels by the use of ammonia and dissociated ammonia.

Single-stage (class 2) nitriding requires a nitriding at 500-525⁰ C and a dissociation rate of 15 to 35%

Two-stage (class 1) nitriding (Floe process) requires a first stage at 500-525⁰ C and a dissociation rate of 15 to 35% and second stage at550 to 575⁰ C and a dissociation rate of 65 to 85%

SAE AMS 2759/10A specifies the nitriding potential to be used instead of the dissociation rate. The specification limits the compound layer thickness in AMS 2759/6 in the same way but adds class 0, where no compound layer is permitted.

Nitriding is carried out in following media:

Gas Nitriding at atmospheric,-low or high pressure in just ammonia with or without addition of nitrogen and or hydrogen. In ammonia with or without nitrogen, oxygen (oxynitriding), sulpher- compound (sulfonitriding)

Plasma as Ion nitriding, DC plasma, Pulsed plasma and Active screen with nitrogen  as processing gas with or without hydrogen or argon

Pack nitriding

Nitrocarburizing processes are carried out in:

Aerated salt baths diffusing nitrogen and carbon into the steel, quenching in oxidation baths and polishing operations in between and subsequently oxidized in an oxidizing bath.

Sulpher containing salt baths.

At atmospheric pressure in ammonia and hydrocarbon or endothermic mixtures, post oxidation, with or without quenching and polishing operations.

Plasma nitrocarburizing adding nitrogen and carbon into the surface.

 

Gaseous Ferritic Nitrocarburizing Treade names :

Nitrotec, Nitemper, Deganit, Soft Nitriding, Triniding, Nitroc, Vacuum Nitrocarburizing, Controlled Nitrocarburizing, Nitro Wear, Corr-i-dur, Oxycad NT

And many more!

 

Nitrex Nitriding and Nitrocarburizing Processes do more or less indicate what it looks like.

Nitreg  Potential-Controlled Gas Nitriding

Nitreg –C Potential-Controlled Gas Nitrocarburizing (FNC process)

ONC  In-process Post-Nitriding/Nitrocarburizing Oxidation

Nitreg  -S Potential-Controlles Nitriding of Stainless Steel

Nano-S   Potential-Controlled Nitriding of Stainless Steel with better corrosion resistance

New Nitreg  -Ti Nitriding of Titanium Alloys

Black-Tride   Post-Nitriding/Nitrocarburizing Oxidation

Plasma (Ion) Nitriding

 

Salt Bath Ferritic Nitrocarburizing Trade names :

Sulfinuz, Sursulf, Tufftride, QPQ, KQ-500, SBN, Nitride,  Nutride, , Melonite, Meli 1, Isonite, Palsonite

And many more!

 

Ion (Plasma) Ferritic Nitrocarburizing Trade names :

Oxinit, Fernit, Plasox, Plastek, Planit, Ultra Glow NC, Nivox LH

And many more!

 

Summary of global offered processes of nitrogen diffusion:

NITROTEC, KOLENE  QPQ, IE.NU-TRIDE, LINDURE,NQ40, NITEMPER, ALLNIT, OXYCAD, TRINIDING,

NITROC, MALCOMIZING, NITRAL, NITREG,CORRIDUR, NITROCARBURIZING, FNC, ANC, NIVOX 2,

NIVOX 4, EXPANITE L, EXPANITE H, SUPEREXPANITE, LOW PRESSURE GASNITRIDING,LINDURE, NAKAL CATALYTIC NITRIDING, OXYNITRIDING, SULFONITRIDING, OXYSILFONITRIDING, PULSED PLASMA NITRIDING, ACTIVE SCREENNITRIDING, NITRAFI, NITROLOX, SOLNIT, IONIT-OX,MALONITE, TUFFTRIDE, ARCOR, NUTRIDE, N-QUENCH, DEGANIT, SOFT NITRIDE, NITRO WEAR, OXINIT, FERNIT, PLASOX, PLASTEK,PLANIT, ULTRA GLOW NC, NIVOX LH, SULFINUZ, SURSULF, KQ-500, SBN, MELI 1, ISONITE, PALSONITE,VACUUM NITROCARBURIZING, CONTROLLED NITROCARBURIZING

 What’s in a name! 

To get what you want and need you should specify, functional properties such as wear-, fatigue- and corrosion resistance.Important properties of the compound layer are: structure, chemistry, hardness and hardness depth.On the diffusion zone: hardness and depth.Steel properties as supplied: alloy composition, heat treatment condition and core hardness before nitriding / nitrocarburizing.

Based on the above the process parameters are to be defined: such as temperature, time, nitriding potential and gas composition.Quality control at the end should verify to what extend the results meet the specification and the required functional properties.Based on the foregoing it would be an advantage when suppliers of heat treatment processes give some more background info to make selection for buyers and design engineers easier.

1). Excerpt of a presentation given earlier at the Nitriding Summit, Wroclaw  8.10.2014

2). Courtesy of  Karl-Michael Winter; Process-Electronic  GmbH 

 

Hans Veltrop

February 22, 2015

 

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This morning I read in my daily paper that it is expected that at this fall a number of casualties will be lost due to CO poisoning in their homes starting up their heating system now again as it becomes colder. Lack of professional maintenance is often the main cause that the heating system is dirty or out of balance with the consequence that incomplete burning of the hydrocarbon gas is taking place producing Carbon monoxide.

In chemistry class on high school we learnt that carbon and oxygen are essential elements that rule our being. We never realized that a difference of just one atom in composition of a chemical compound may be a healthy or a deadly issue.

CO is: colourless, poisonous, very inflammable, some lighter than air, odourless and tasteless, large diffusion power and in air mixture explosive. All IQ’s, chamber- and pit furnaces in which CO containing process gases are used, leak CO into and raise CO content in the working area when not adequately ventilated, caused by:

  • Gas burners not correctly adjusted and not frequently checked.
  • Leakage at doors, explosion lids, pipe work couplings and extinguished pilot burners.
  • Bad cleaning procedures/ oily loads in tempering furnaces increase CO pollution substantially.
  • No or badly functioning venting systems.
  • Particularly in winter when windows and doors are closed.

International regulation is diverse concerning what is allowed in working areas.

Mac-values which might be slightly adjusted meanwhile are:

  •   0-1ppm                          normal presence
  •      9 ppm                          norm for domestic areas
  •    30 ppm ( Germany)     confined spaces, average 8 hrs max
  •    25 ppm (Netherlands) allowed value
  •    50 ppm (USA)              no uniformity yet

In case of the following CO values, actions to be done and consequences are:

  •  100 ppm                           personnel shall be evacuated
  •  200 ppm                           light headache, tired, queasy and dizzy
  •  800 ppm                           dizzy, queasy and cramp; dead within 2-3 hrs

In case of peak values to stay in an environment is limited:

  •  Max 150 ppm                   during 15 minutes
  •  Max 120 ppm                   during 30 minutes
  •  Max   60 ppm                   during 60 minutes

The above values mean in fact that workers on the shop floor when in case of higher CO values are exposed to working conditions that are unhealthy and that their functioning may be less alert and might lead to wrong decisions. As such it is not only a health & safety issue but an economical and quality issue as well.

Fact is that circumstances which cannot be tolerated and in practise occur have been shown by an international investigation on working conditions in western state of the art heat treatment shops of which 40 % showed high and far too high CO contamination in the working zone.

The reason that these circumstances occur is the sneaky properties of CO and in many cases is not recognized by the management of heat treatment shops. To get insight in this phenomenon is simple by installing cheap analysing equipment in the vicinity of heat treatment equipment, measuring and monitor the CO ppm in the air by which corrective actions can be taken.

I remembered the results of this investigation when I read a column on the site of the “Monty” in June in which the question was raised on the future of heat treatment and to what extent the Japanese model should be followed, in fact the discussion on furnace emissions.

Because of the importance of the issue, I copy the article in full with permission of “The Monty”:

The Future of Heat Treating (will we be following the Japanese model)? Over the years we have asked a number of very experienced heat treaters what they expect the industry to look like in the future. While we have heard a number of ideas none of them (with all due respect) were very definite or “earth shattering” for the obvious fact that none has a crystal ball to the best of our knowledge. A fellow the other day though did have a very definite opinion, an opinion which we didn’t particularly like but one which we feel is quite possible furnace emissions. The current almost cult like, fanaticism about greenhouse gas emissions causing global warming (or global changeor whatever it seems to change daily) shows no sign of abating in the near future and the recent decision by the US to work towards closing down coal fired power plants is a perfect example of this (we do agree that reducing pollution is a very worthwhile cause but greenhouse gases are another story in our humble opinion). A recent visit to Japan showed that heat treaters there are constantly trying to reduce all forms of furnace emissions and energy usage with the result that most of the atmosphere furnaces we saw have vacuum purge chambers on both the load and unload stations specifically to trap all emissions. While Japan has taken the largest steps towards this no issue parts of Europe appear to be going this direction also leaving North and South America as the outsiders. All of this takes us to this point; will North Americans be forced within the next few years to be equally as concerned with reducing green- house gas emissions? Far fetched as it might sound our understanding is that one of the largest captive heat treaters in North America has already mandated that all atmosphere furnaces will be shut down within 5 years to be replaced by vacuum carburizing units. We at “The Monty” believe that the only way emissions can be reduced are by switching to vacuum furnaces, expensive modifications toatmosphere furnaces or by converting all gas fired furnaces to electric which of course only pushes the issue down the road to the electrical suppliers. Perhaps it might be far fetched but it is also possible that within a few short years this will become a major issue and we in North and South America will all be looking towards Japan and Europe for suggestions. While we would find this an incredible waste of time and money stranger things have certainly happened. The future? Perhaps. June 26/2014" End of the Monty article

 

Although I fully endorse the opinion of ‘The Monty”, I am the opinion that a difference should be made between emissions due to energy usage and atmospheric related emissions.

The solution on energy emissions are still a long way to go whereas emissions due to process atmospheres can be easily accomplished by evacuating before loading and de-loading as described above, but I expect after many attempts in the past, developments recently revealed by S.Bischoff of ROHDE Schutzgasöfen GmbH in Germany in cooperation with IWT Stiftung Institut für Werkstofftechnik,Bremen-Germany but also a promising project of Air products and Chemicals Ltd and inventer Zbigniew Zurecki et al, are quite promising to reduce CO₂ and CO emissions from furnace atmospheres by absence of oxygen in mixtures of just nitrogen and hydrocarbons at the same time reducing IGO.

After all it is remarkable that a difference of one atom of oxygen in a molecule CO which by lack of one oxygen atom causes that our blood is incapable to transport the oxygen to our vital organs and may cause dead, whereas an additional oxygen atom in the molecule CO₂ is needed to bring us so much joy drinking beer with a nice foam on it or making champagne sparkling and to finish this blog on a technical manner CO₂ is used to stop a fire; very remarkable but we have to live with it.

Hans Vetrop

November 4, 2014