Drill rods for geological operations consist of one end threaded internally to form a pin, and the other end threaded externally to form a box. The 3.1m long rods are fit together to provide the length needed for drilling. Originally, these AISI 4130 steel drill rods were imported. It was then decided to manufacture these locally by threading rods imported from India. These rods began seizing prematurely in the field.
Pin
Box
Objectives
One Eighty was required to determine the root cause of failure of the drill rods
INVESTIGATION
Two batches of rods were imported from India. Batch 1 has been threaded. A sample of the original rod and one from each batch from India were sent for spectrographic analysis to determine material composition.
Samples from the original drill rods and both batches from India were sectioned for metallographic examination and hardness testing. In the case of the original rods, samples were taken from the pin and box threads and the base material away from the thread. For Batch 1 samples were taken from the thread and the base. For Batch 2 a sample was taken from the base.
WHAT WE FOUND
RESULTS
Spectrographic analysis confirmed that the original rods and the two batches from India are AISI 4130 steel.
Average results from the Vickers hardness tests are shown. There are a number of different tempering treatments which can be applied to the steel in order to give the required properties. As a means of comparison, the hardness values for AISI 4130 steel under various heat treatment conditions are listed.
Hardness Results
| Sample |
Hardness (HV) |
| Original Pin Thread |
320 |
| Original Pin Base |
255 |
| Original Box Thread |
263 |
| Original Box Base |
269 |
| Batch 1 Pin Thread |
290 |
| Batch 1 Pin Base |
290 |
| Batch 2 Base |
255 |
Hardness Results for various Heat Treatments
| Heat Treatment |
Hardness (HV) |
|
R |
208-243 |
|
S |
238-294 |
|
T |
260-318 |
|
U |
286-350 |
|
V |
310-372 |
|
W |
327-397 |
|
Normalised |
228 |
In the case of the original rods, only the pin thread has been hardened, and there is a substantial difference between the base and the thread values. There is no noticeable difference in results from the thread and base material of the box, and these results are similar to the base hardness of the pin.
The sample from Batch 1 shows no hardness variation between the thread and base material. This is as expected, since this rod was threaded locally and no heat treatments were applied after this.
There is a difference in hardness between the two batches of rods sent from India.
The microstructural analysis supports the findings of the hardness measurements. The original pin thread has been quenched and tempered, resulting in tempered martensite, which is responsible for the high hardness of 320HV. This indicates that Heat Treatment U or V has been conducted. The pin base material is in the normalised condition, indicating that the heat treatment was confined to the thread. The box thread and base material are also in the normalised condition, as expected.
The relatively higher hardness of the Batch 1 samples is explained by the tempered martensite structure. This indicates that Heat Treatment S or T has been conducted. This tempering treatment does not match that of the original pin thread. This means that there is a risk that Batch 1 may not give the same performance.
The Batch 2 rods have not undergone heat treatment.
Batch 1 Pin Thread showing Quench and Tempered Condition S/T
A summary of the pin and base material conditions for each batch is shown.
Box and Pin Material Condition
- The original rods, as well as both batches from India, are AISI 4130 steel, as specified.
- In the original setup, only the pin thread has been hardened by quenching and tempering. No heat treatment has been applied to the base material and box thread, which remain in the normalised condition.
- There is a difference in hardness and microstructure between Batch 1 and 2. The sample from Batch 1 has been quenched and tempered, resulting in a higher hardness, while the batch 2 samples are in the normalised condition.
- Premature failure was caused by hardening treatments that were not consistent with those applied to the original drill rods.
