Spotlight – How to... find heat treatment data for a specific steel grade

Materials World magazine
,
1 Jun 2018

Joakim Fagerlund*, Senior R&D Engineer at steel solutions company Ovako, Sweden, talks about the company’s new online heat treatment guide.

When choosing the right steel for an application, there are many factors that need to be taken into consideration. Typically, these are hardness/strength and hardenability (hardness as a function of quenching rate). But, there are many other factors that can also apply, such as ductility, density, hardness after nitriding/carburising, corrosion resistance, cold formability, carbides (size and distribution), cleanness (size and distribution of inclusions), price/alloying content, electrical properties, and weldability.

As there are many steel grades available worldwide, there is generally a lack of reliable heat treatment data for a specific grade – especially for less common and customer specific grades. This has prompted Ovako to develop a tool that predicts hardenability and hardness as a function of tempering temperature.

The concept behind the tool is that it should be valid for all steel grades, such as low-alloyed steels, engineering and special steels (quench and tempering, carburising, nitriding, and bearing steels), stainless steels, tool steels, and precipitation/age hardenings steels, such as maraging steels.

The model

The software is based on several neural network models, one for hardenability – continuous cooling transformation (CCT) diagram shows the types of phase changes that occur in a material as it is cooled at different rates and Jominy-hardenability – a test to determine the hardness of materials – and one for tempering response (diagram). The models are based on both Ovako’s own internal data and from available literature. The hardenability model is based on approximately 97,000 individual measured data points. The tempering model is less extensive, and based on around 6,000 data points. It is based on a short tempering time of around one-to-three hours of a perfect quenched microstructure – the hardness is assumed to be as high as possible.

Input data

The input data needed to predict the hardenability and tempering response is the alloying content (Fig 1). The austenitisation (heating an iron based metal to a temperature at which it changes from ferrite to austenite) time and temperature is assumed to be suitable to give as high hardness as possible after a fast quench. The table shows the alloying elements that are included in the hardenability and tempering model are shown. For each model and element, the range is shown for which the model has been created. The user enters the desired chemical composition, or can select one of approximately 1,000 predefined steels grades on the list.

Output information

When the user has entered the chemical composition and pressed the calculation button, the results are presented in several graphs and tables. The hardenability is presented as a CCT-diagram (Fig. 2), Jominy-curve, and as a table where core hardness is presented for various bar sizes that have been cooled by water, oil, and air. The effects of tempering are presented as a graph of hardness as a function of tempering temperature (Fig. 3) and a graph of strength as a function of tempering temperature (Fig. 4). If the user wishes to compare several steel grades, the results are presented on the same graph for ease of comparison.

To illustrate the effect of different elements on the hardenability, the steel grade 1040 (0.4 carbon, 0.2 silicon, 0.7 molybdenum) was used as a base. A calculation was performed on the base alloy and compared with five alloys, which had an addition of 2% of silicon, manganese, chromium, nickel, and molybdenum to the base composition. In Fig. 5 the results are presented for the base alloy and the five variants. The results of these calculations show that molybdenum has the strongest effect on the hardenability followed by chromium and manganese. Silicon and nickel have the least effect on the hardenability.

Limitations of the tool

The main limitations of these models are that the austenitisation time and temperature is not a user input. The prior grain size is also a factor that is not considered in these models. There are also alloying elements that influence the results that are not included, such as nitrogen, niobium, phosphorous, and sulphur. The tempering time is not a user input and it is assumed to be short – approximately one-to-three hours.

Despite these limitations, Ovako believes that the results from the calculation can be accurate enough for most cases and the tool will give a good indication of how different steels behave in the quench and tempering processes.

In summary, this is a general tool that can predict the effect of quenching and tempering on steels. The model is free and open for all to use online. The guide can be found at bit.ly/2jNKasw

*Joakim Fagerlund is a Senior R&D Engineer in Ovako’s R&D department in Stockholm, Sweden. In his current role, he focuses on development related to steel cleanness and fatigue.

Improving stainless steel

An advanced process for low-temperature surface hardening of stainless steel that allows for accurate tailoring of material properties has been developed by the EU-funded project PlaSSteel. It can be applied to ferritic, martensitic, austenitic, and duplex grades of stainless steel, increasing surface hardness by up to four times, as well as improving adhesive and abrasive wear.

The process is based on plasma technology, comprising a nitriding and nitrocarburisation process below 500oC, enriching the surface of the material with nitrogen and carbon. The gas used in the nitrocarburisation can be methane, propane, or natural gas, with content between 2–10%. 

Many surface-hardening techniques are known to diminish the original corrosion resistance of stainless steel. The project, however, has found a solution through the use of IONITECH’s plasma nitriding furnace. It provides precise control over the material properties, and allows for working at low temperatures, remedying the need to dissolve nitrogen or carbon into stainless steel without forming chromium nitrides or carbides.

The project used a plasma nitriding/nitrocarburising furnace to achieve uniformity of heat over the material surface. ‘The novel plasma nitriding furnace eliminates the possibility for the hollow-cathode effect,’ stated IONITECH leading research and development specialist Alexander Varhoshkov. ‘This local overheating might lead to temperatures above those needed for the PlaSSteel process that will in turn lead to chromium carbide and chromium nitride precipitations on the grain boundaries of the steel. These areas will have higher surface hardness but will also be susceptible to intergranular corrosion.’

A number of stainless steels were processed and tested for the project, giving varied results based on the alloying elements. Some elements made the diffusion of the carbon atoms more difficult than with others, which led to minor differences in the diffusion layer and surface hardness. 

The Community Research and Development Information Service (CORDIS) believes that the project will lead to an increase in the competiveness of the EU stainless steel industry, as well as improving the quality and safety of steel products. 

Bodycote gets Rolls-Royce deal

A £160m, 15-year deal has been reached by engineering group Bodycote and Rolls-Royce, UK, to help in the creation of a new line of civil aerospace engines. Bodycote will supply its thermal processing services in the form of vacuum heat treatment and hot isotactic pressing.

The processes will be used to strengthen the metals and alloys used in the turbine blades, extending operational life. This will cover the Trent XWB, 1000, 7000, 700, and 900 engines.

In a statement, Bodycote said, ‘Bodycote’s core business is to provide services that protect and improve the properties of metals and alloys, extending their operational life and making them safer.’

Carbolite Gero upgrades HTFs

Carbolite Gero has recently re-engineered a range of floor-standing high-temperature industrial chamber furnaces (HTFs) from its catalogue, providing models ranging from 64–514l, with a maximum chamber temperature of 1,600oC–1,800oC. 

The furnaces use free radiating molybdenum disilicide Kanthal Super heating elements to allow for rapid heating, which lends the furnaces to heat treatment applications in the technical ceramics, semi-conductor, and metals industries. 

Digital controllers can also be fitted to the HTF range, along with multi-segment programmers and data loggers.