3D Nitriding Lab

Gas nitriding is a surface hardening process, where nitrogen is added to the surface of steel parts using dissociated ammonia as the source. Gas nitriding develops a very hard case in a component at relatively low temperature, without the need for quenching.

This lab will demonstrate how to use MO template to prepare a Nitriding simulation. The thickness of compound layers formed on the surface of pure iron during the nitriding process was analytically calculated. Two separate equations were applied for predicting the thickness of the binary compound layers; epsilon () and gamma prime (), in terms of the nitriding process parameters.

1.1. Creating a New Problem

1.2. Adding Operation

1.3. Convert 2D Mesh to 3D

1.3.1. Geometry type

1.3.2. Configuration

1.3.3. Workpiece

1.3.4. Mesh

1.3.5. Material

1.3.6. Convert

1.3.7. Generate DB

1.4. Setting up 3D Nitriding Operation

1.4.1. Simulation Controls

1.4.2. Material List

1.4.3. Workpiece

1.4.4. Initialize Volume Fraction

1.4.5. Boundary condition

1.4.6. Stopping Controls

1.4.7. Step Controls

1.4.8. Generate Database

1.5. Running Simulation

1.6. Post Processing

Creating a New Problem

On a Windows machine, go to the button select DEFORM-v1x.xxx (.xxx indicates version number E.g. v14.0.2) and select DEFORM GUI Main v1x.x from the menu. The DEFORM GUI Main window will appear as shown in Fig. 3DNL1.1.

DEFORM GUI Main window

Create a new problem either by selecting File New Problem or by clicking the New Problem icon. The Problem Setup window will appear as shown in Fig. 3DNL1.2. Select “ Integrated Manufacturing Process “ radio button and unit system as “SI “ radio button in unit field. Define Problem Name as “ 3D_Nitrding_Lab1 “ and make sure the “Show option dialog” check box is turned on (if we do not turn on the “Show option dialog ” check box, then we will not get the New Project dialog in MO UI). Then click on button to open a new Problem using the Deform Integrated Manufacturing Process.

New Problem page

Multiple operation wizard will open with the New Project dialog, at this point user will be prompted to specify a project name (system will create a separate folder with this project name) and title for this session. In this session we will use “3D_Nitrding_Lab1 “ as the project name. Click on to continue to open the operation.

Adding Operation

Add a ‘2D to 3D Converter’ and a 3D Forming operations from operations explorer, in this lab 3D object is generated by extruding the 2D mesh prepared previously in the 2D Nitriding Lab (See Fig. 3DNL1.3.).

Added forming operation into operation editor

Convert 2D Mesh to 3D

Click on the first operation ‘2D to 3D Converter’, to generate the 3D meshed object for the Nitriding simulation’.

Geometry type

Turn on ‘2D Plane strain ’ radio button in geometry type page, see Fig. 3DNL1.4. Then click twice to navigate to ‘Configuration’ page.

Plain Strain Geometry type selection

Configuration

Type 1 for the ‘Length (Fig. 3DNL1.5.). Then click , leave the default one object, click go to ‘Workpiece ’ page.

Configuration for Extrude Type Object

Workpiece

Click on , browse to the file ‘ 3D/LABS/Nitrding/2D_Slab_Specimen.key ’’, (import the 2D mesh generated in the 2D Nitriding Lab 1) and import it, observe the object name ‘Cold-rolled slab specimen ‘, set the temperature to 570 °C and change ‘Object type’ to ‘Plastic ’(see Fig. 3DNL1.6.)

Preview of the imported 2D Workpiece

Click twice to navigate to ‘Mesh’ page, the ‘Geometry’ is not required for this lab, hence it has been skipped.

Mesh

Click “Hexahedron “ and uncheck “Remesh “ check box to keep the same 2D mesh as the cross-section. Type 5 into ‘# of Mesh Layers ‘, this will bring the total number of elements to 41155. Now click to Material page. (See Fig. 3DNL1.7.)

Workpiece Mesh page

Material

Click on , browse to the file ‘Iron_SI.key ’ form 3D/LABS/Nitrding/ folder, to import ‘Iron’ ( Fig. 3DNL1.8.) mixture material. Click to go to ‘Convert’ page.

Material (Import) Selection

Material Assignment Window

Convert

Click on to generate the 3D mesh, the mouse icon on the screen turns to busy, also a bar will show up at the right bottom indicating the converting progress.

In the end, mouse icon goes back to normal, ‘Conversion succeed …’ messages can be seen on the bottom of the window too, which indicates the mesh has been generated, the results are also displayed on the central graphic area, see Fig. 3DNL1.10. Click to Generate DB page.

Preview of Generated 3D Object

Generate DB

Click on to generate the DB which contains the 3D mesh. Now the 3D object is ready and can be passed to the next Nitrocarburizing simulation, ‘Convert 2D to 3D’ operation has been completed.

Setting up 3D Nitriding Operation

Now click on the second operation ‘Forming’, a window will pop up (Fig. 3DNL1.11.), click on ‘YES.’ Select ‘Interactive Setup ’ to continue.

Setup Type Pop-up

Type the name tag of the second forming operation, change it to ‘Nitriding ’.

Simulation Controls

In this lab, we will be demonstrating how to setup Nitriding process which requires some advanced options here, Switch to mode in ‘Simulation controls’ window make sure ‘Diffusion ’ and ‘Transformation ’ models under ‘Heat transfer ’ are checked (See Fig. 3DNL1.12.). Then click on ‘Process conditions ’ page.

Simulation controls window

Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface, so in this lab, we will deal only with atom – nitrogen. Click on “Process condition “ then ‘Diffusion ’ tab, change the atom’s name from ‘carbon’ to ‘Nitrogen’ (See Fig. 3DNL1.13.). Then click . Click in popup.

Modify atom’s name in simulation control window

Material List

Iron has been added to the lab in the previous operation and assigned to object already. Now let’s review the material data. Compound layers (+) will form on the surface of pure iron (). So, in the material page, it can be observed that ‘Multiphase Mixture material’ is checked for Iron and it has three child phases: Alpha , Gamma-prime , and Epsilon.

In the nitriding of iron, when the nitrogen concentration exceeds the solubility limit, extra nitrogen atoms make stoichiometric compounds with iron atoms. The surface composition of the nitrided iron can be predicted by considering the Fe-N binary phase diagram. In this lab demonstration, the surface structure of the nitrided iron includes -Fe (N) diffusion zone (solid solution of nitrogen in -Fe), and compound layers. The nitrogen contents (solubility limits) at the material interface are listed in table 1.

Position	N Content (At. Pct. N)
Surface	26.34
/	23.59
/	19.923
/	19.479
/	0.365

Nitrogen content

Click , comes to the Iron’s material page. For this Nitriding lab properties like phase transformation and diffusion coefficient are required.

Transformation

To check the phase transformation relationships of Iron, click . In ‘Transformation’ page, there are two defined relationships. The first is Alpha Gamma-prime. Under the ‘Kinetics’ tab, select ‘Diffusion (Solubility curve)’ from the pull - down list to model the transformation (See Fig. 3DNL1.14.).

The nitrided layer thickness growth of , follow the parabolic law, select the following model for the layer (Alpha Gamma-prime)

and 0.6 is set for the correction factor K. At 0.365, the solubility of nitrogen, Alpha starts transforming to Gamma-prime. In the end, the nitrogen content will reach 19.479, and Gamma-prime forms totally. So, at Alpha/Gamma-prime interface, nitrogen content ‘Start’ value is set as 0.365, and ‘End’ value 19.479 (See Fig. 3DNL1.14.).

‘Material Editor’ - transformation definition 1

“Add another transformation” by choosing mother phase as Gamma Prime and child phase as Epsilon. Choose the layer growth model for (Gamma-prime Epsilon) as listed below, K has been set to 0.3. And the solubility of nitrogen starting from 19.923 to the end 23.59 as Gamma-prime transferring to Epsilon. (See Fig. 3DNL1.15.).

‘Material Editor’ - transformation definition 2

Diffusion Coefficient

The diffusion coefficients for all the phases are listed in table. 2, click the icon on the material page to check the values.

Nitriding Temperature [°C]	570
Diffusion Coefficient of Nitrogen [10 -8 mm2/s ]	Epsilon ()	3.4
Gamma -prime ()	18.1
Alpha ()	983.3

Diffusion coefficient of nitrogen

Thermal Properties

Thermal properties are not necessary because the object’s temperature is constant and same as the environment temperature in this lab. But they are still required for DB generation. For Iron, 30 has been defined for thermal conductivity, 5.5 as heat capacity, 0.7 as emissivity, and 7.85e-09 as density. Same values are also defined for all the child materials.

Thermal Properties Page

Workpiece

Object ‘Cold-rolled slab specimen ’ has been imported from the previous 2D to 3D Converter operation. Observe from the ‘Navigator’ window object information like mesh, material are listed.

Initialize Volume Fraction

At this moment, click on to access the element dialog to initialize the volume fraction. On the item list window click ‘Microstructure’‘Phase’. Then choose ‘Alpha ’ and click on to ‘Initialize Element Data’. Type in 1 , then click on , then close the window. click on until BCC page.

Element Dialog – Initialization of Phase Volume

Boundary condition

Heat Exchange with Environment

Click on “Heat Exchange with Environment BCC” then click on “Environment “ to change the ‘Environmenttemperature ’ to 570 °C, which is same as the object temperature (See Fig. 3DNL1.18.).

Heat Exchange with Environment Definition

Diffusion BCC

Constant Nitrogen contents on the workpiece are assumed in this Nitriding Lab simulation, to do so click on “Constant Nitrogen Surface Content”. Then type in 26.34 for the ‘Atom Pct.’. Use the mouse, pick the surfaces except +Y and -Y surface as they represent symmetry (See Fig. 3DNL1.19.), then click on to finish the assignment. Click until Stopping controls page.

Constant Nitrogen Surface Content

Stopping Controls

Make sure the system is in ‘Expert’ mode, if not, click on will switch the system to the expert mode. Then type in 36000 in the ‘Process**duration** ’ field, see Fig. 3DNL1.20. Then click on to “Step controls” page.

Stopping Controls (Expert Mode)

Step Controls

Switch back to the ‘Guided’ mode by clicking on , Since process duration has been defined simulation will stop accordingly, type 999999 into ‘Numberof steps ’ field. Set 5 as ‘Stepincrement ’ and 20 sec. as the time per step (see Fig. 3DNL1.21.). Then click on to ‘Generate DB’ page.

Step Controls (Guide Mode)

Generate Database

In ‘Generate DB’ page, click to see if anything was missed in the setup and then click on the button to generate the database. Observe the messages in Message tab informing database generation status.

Running Simulation

Once the database has been generated, switch to the Simulation mode by clicking on button above the operation tree. Click on the action label to open the Run Options dialog as shown in Fig. 3DNL1.22. Use the default Continue Run option to select “Continue from the last step ” (from step -1) option and then select the Simulation mode as Interactive and click on button to run the simulation.

Run Simulation Window

Monitor the progress of the simulation by looking at the Simulation Message and Simulation Log tab, making sure that the option is checked. User can view the Nitriding process as the simulation proceeds to the specified Step definition from Simulation graphics.

Post Processing

After the simulation is finished, open the DB in Next Gen post - processor.

Nitrogen Profiles

‘State variables between two points’ function is a great tool to exam nitrogen concentration profile (vs. depth below the surface).

Click on , Under Diffusion Dominant atom, select “ Nitrogen “ State variable and click on to plot and click on .

Go to last step, then click on State variables between two points to generate Nitrogen profile. Define Start and End points and click on generate (see Fig. 3DNL1.23.).

SV between 2 Points: Atom-Nitrogen