Heat Treatment TTT diagrams

RONIN Custom Knives, Daniel Gentile (© 2005) Heat Treatment for Bladesmiths & Knifemakers A Modern & Practical Approach Chapter 1.8 Theory: Understanding TTT Diagrams [Fig. 1] TTT Diagram for C45 (1045) Steel Austenitizazion Temp: 880°C Optimal Heat up time: 2min Holding Time: 10min Hardness in HV10 Legend: A = Stable Austenite Range F = Ferrit range P = Perlite range B = Bainite range M = Martensite range M s = Martensite Start-Temp. The TTT (Time Temperature Transition) Diagram shows the required holding time, temperature & the cooling rates for specific heat treatment of a specific steel. These Diagrams are not universal. Normally A Steel Sup- plier should be able to hand out the TTT Diagram for (almost) all of his Steels. In Fig. 1 is a TTT Diagram for 1045 Carbon Steel. As mentioned earlier in this document (Chapter 1), basically heat treatment of steels should be regarded as the accurate technique to change the „Structure“ of the steel... Most of the times a successful heat treatment process of carbon & tool steels will lead to a steel in it‘s martensitic state. As this can be seen as it‘s hardest state, thus enabling it to hold an edge. Sometimes Bainite is another ideal structure to go. The TTT diagram can show you how long a specific Steel needs to be held at a specific temperature to com- plete it‘s austenitization. And it will reveal the required cooling rate to form the desired structure. By understanding the TTT diagram you are able to achieve a very specific, optimal result. Sure heat treatment can work without all the bells and whistles, without pyrometers, controlled furnaces, magnets and other toys... You simply can watch for the „Shadow“, the visible side effect of the physical phenomenon of recalescence and decalescenece, remember the correct colour and perform the quench in the appropriate medium. It works, and with some practice you will get good results. Whilst this method can work well, with a lot of experience, for simple carbon steels which do not require long holding times to achieve a complete austenitization, it will be a very difficult task for any type of stainless tool steel. I believe in achieving optimal results each time. Optimal results literally translates to a controlled work proc- ess which has been optimiztied to reduce failures and unwanted / unnoticed changes to a minimum. To use the information provided in the TTT diagrams there is almost no way around modern equipment. Whilst a „successful“ heat treatment process can be achieved even with temperature differences of +/- 100°C it will, that way be far from what the technical optimum would be. Studying the TTT‘s shows some of the complexity of the whole process, especially when you consider that 1045 is a pretty simple steel to heat treat and will forgive a lot of mistakes. Steels like ATS34 require working within a close range of optimal temperatures and time (+/- 10 °C). When I made my first encounter with the TTT‘s my first thought was to dismiss them as a scientific nonsense diagram no one will really understand. They look complicated, and do not welcome you with a glass of wine and some nice dinner. So many just forget about them. It took me some time to understand, that a TTT diagram is actually easier to read than it looks like. The easiest way to approach the understanding of TTT‘s is to separate their constant and look, at least for the first time, at them one by one. Once this is clear, put the jig-saw back together and it should start to make some sense. First we should give the X and Y scale some attention. The Y scale shows a temperature range, starting at 0 by it‘s lower left corner. The Unit system (°F / °C) is usu- ally indicated at the top left corner. The X scale shows a logarithmic time scale. The reason for the complicated looking log time scale is actually a simple one: It allows for very short and very long times to be displayed in the same „small“ diagram. Look only at the scale and it is quiet simple... it progresses in seconds. Some TTT diagrams use a exponential view (10 1 10 2 10 3 10 4 ... Seconds) rather than writing the whole long zero-filled number... but it‘s basically the same so that should not be too confusing. Next: The curved lines (transition lines) and A 1 & A 3 mark-lines This curves show various holding times at a given temperature and cooling rates. The Temperature range where the austenitization occurs is displayed within the margin of the horizontal Lines A 1 & A 3 , with A 3 being the exact austenitization temperature for the given steel. We will go into detail of the curves in Fig. 1 in a moment... And last but not least: The thick, darker limes and ranges They divide the Diagram into the various Steel-Microstructures (Ferrite, Perlite, Martensite, Bainite, Austen- ite). Put all together again and try to interpret it for the 1045 example: We have A3 at approx. 880°C (the exact temp. is usually listed in text form above or below the diagram). Vickers & Rockwell HV HRb HRc 193 90 - 205 92.5 - 250 99.5 22 280 - 27 302 - 30 527 - 51 544 - 52 773 - 63 If we follow curve 1 we will notice that after approx. 130s at a temperature of 720°C we will cut the first transition line (the time count begins just after a spe- cific transition line has reached Ac3). So this is the point where the steel starts to form Ferritic-Crystals at the Austenitic-corn boundaries. Just after 300 seconds the transition line crosses the Perlitic range. After 330s the perlite formation is completed and the steels microstructure and consists of 50 %-vol. of Ferrite and 50%-vol. Perlite with a Hardness of approx. 200 HV10 (Hardness Vickers). The steel is really „soft“ in this state. The Transition curve 2 marks a faster cooling compared to curve 1 as just after 4.5 seconds the pre eutektoide ferrite formation starts. And after about 8 seconds perlite starts to form. It does not take more then 10 sec to complete the whole transformation and the Micro- structure now consists of 25%-vol. Ferrite and 75%-vol. Perlite with a hardness of 274 HV. The Cooling rate for transition curve 3 shows the start of the ferritic transformation at about 650°C after 2.3sec. After 3.8 seconds Perlite starts to form and after just 6 seconds Bainite. Finally after 16 sec at 285°C the left amount of Austenite begins to transform to martensit. After the steel has cooled to room temperature the microstructure consists of 10%-vol. Ferrite, 80%-vol. Perlite, 5%-vol. Bainite and approx. 5%-vol. Mar- tensite with a hardness of approx. 300 HV. Curve 4 is interesting due to it‘s bainite & martensit results... The cooling rate shown for curve 4 shows the beginning of the Bainitic transformation just after 2.2 seconds and the transition to martensite after 3.9 sec . As soon as the steel has reached room temperature its „internals“ look like this: 2%-vol. Bainite and ~ 98% Martensite, which results in a hardness of approx. 548 HV (~52 HRc). And at long last curve 5: With a cooling according to curve 5 the austenitic structure at first remains stable until it has reached 340°C (within ~2.5 seconds (!)) and then the formation of martensite begins. When the steel has cooled to room temperature in little less then 10 seconds it‘s microstructure consists of pure martensite at 760 HV (~ 61HRc). If we now go over the practical meaning of the process of turning a 1045 unhardened blade into a heat treated knife blade (with a martensitic structure) the TTT diagram reveals the following very important information: 1) The austenitization temperature is at 880°C 2) The holding time is really short (it‘s actually almost nil) (however I have to mention that a holding time is considered a „holding time“ only as soon as the whole steel is at the desired (880°C) temperature... so this varies for with the actual thickness of a specific material) 3) To form „pure“ martensite the Blade needs to be cooled to 340°C within 2.5 seconds. (This helps choosing the quenching medium, and temperature) 4) As soon as it has reached 340°C there is not much need to rush things... (which helps to realize that actually a lot of post-quench stress can be avoided with using the proper quenching medium) Interpreting the TTT diagrams can take some practice but it‘s worth the effort. It will reveal the steels secrets to you and you will not regret the time you have spent on studying the diagram as it will enable you to improve your results and avoid (sometimes fatal) failures.