Immersion samplers are used to collect samples of molten metal during the different steps of processing. The samples are used in chemistry analysis to verify the proportional elemental make-up, confirm the absence of contaminants, and to ensure quality control of the final product. In iron and steel production, it is common to use immersion samplers to collect data about carbon content as well as alloying elements for the production of specialty alloys.
The importance of measuring these quantities in different circumstances has led to the development of different styles of immersion samplers. This guide will highlight the strengths and uses of each type of immersion sampler.
To reap the benefits of alloying elements and ensure quality control of each heat, metallurgists must be able to sample the liquid metal for analysis. Immersion samplers are used to take samples of molten metal for chemical analysis using spectroscopy, combustion analysis or other means.
Samples must be homogeneous, meaning that their chemistry must be consistent throughout. With the wrong choice of sampler, it is possible to contaminate the sample with dross or slag, which will embed oxides and other contaminants in the sample. This is particularly true if the sample collection method is too turbulent and pulls the lighter oxides deeper into the heat.
In order to get useful chemical data, samples must be collected in a standardized manner so that each sample can be compared with historical data. From there, metallurgists can identify trends, such as contamination from eroding furnace lining, master alloy components and other sources. Also, multiple samples can be collected from a single heat to verify homogeneity throughout. If some samples show different chemistry, it is possible that segregation of some elements or alloys is occurring and melting parameters must be changed.
Certain elements, such as silicon and copper, should be tracked carefully in iron and steel alloys. Copper contamination in steel can cause it to lose ductility and can also raise the ductile to brittle transition temperature. Silicon has some tremendous advantages in steel and cast iron alloys, but too much will lead to reduced hardness and ductility as well. It has a tendency to form graphite flakes instead of nodules (gray iron to ductile iron, respectively), which reduces fracture toughness and ductility significantly.
There are several common immersion sampler types: round, oval and twin. They are also available in several standardized sizes, one for each shape. The trick is choosing a sampler shape that matches the analytical tool that will be used.
Round samples are sometimes called “spectroscopy disks” as they are frequently used for spectroscopy. Immersion samplers have an advantage over other sampling techniques as they can produce uniformly-sized disks that require minimal preparation before analysis.
The round samples have the advantage in their ease of use. There is no special alignment, and the same sample can be tested in a few different places by simply rotating it.
Oval immersion samples are used in spark spectroscopy due to their increased likelihood of homogenization. Internally, oval samples allow for better mixing of samples than round samples, and thus are preferred for some analyses. Also, they have larger surface area to volume than round samples, which means there is a better atomization during spectroscopy, and thus a stronger spectrum, meaning better sampler accuracy. However, these advantages are slight over round samples.
Twin immersion samplers are sometimes called “dual thickness” or “stepped” samplers, as the sample is of two separate thicknesses. If the two thicknesses have different compositions, this is an indication that thin, cast sections may be a different composition. This could be due to a number of issues that will depend on the specific alloy. Furthermore, the two thicknesses will cool at different rates, and so the effects of cooling rate can also be demonstrated.
Twin immersion samplers can also be used to generate calibration samples for some analytical equipment.
Foundry sampler selection relies on several criteria: composition, sizing, and reusability.
Immersion samplers are constructed using two halves of a mold which are assembled together in a sand cast cartridge including a quartz glass tube to deliver the molten metal into the mold. The glass collection tube is protected by a metal cap that melts on immersion to expose the tube. After collection, the sand cartridge and shell is broken open and the sample is sent to the lab. Often, a pneumatic tube delivery system is used to quickly deliver samples from the melt deck to an onsite lab for immediate analysis.
Immersion samplers are available with several different deoxidizer materials inside the sampler shell. The purpose of the deoxidizer is to capture and sequester any existing free oxygen, thereby keeping the sample from becoming oxidized and falsely affecting the chemistry, and to prevent the formation of gas bubbles, which can give an artificially high porosity content.
Standard deoxidizers are aluminum, titanium and zirconium, which are typically used with steel and high temperature alloys. Samplers can also be purchased with no deoxidizer at all, as is typical for most grades of iron.
All three immersion sampler shapes can be assembled inside the same 1.53” (39 mm) standard tube size. The choice of tube lengths range from 32”, 44” and 72”, with custom sizes available. The choice in length is dictated by the equipment and configuration of the foundry where the samples will be collected. Longer sample tubes are convenient for keeping foundry workers a longer distance from the molten metal or for when samples are required from deep within the furnace.
Immersion samplers are considered a consumable part since they are used only once and must be broken open to retrieve the sample. However, samplers and sampler tubes can be purchased fully assembled or as separate components. If purchased separately, the tubes can be used twice by inserting a cartridge and taking a sample at one end, then turning the tube around and using another cartridge at the other end. Fully assembled single-use samplers and tubes make the melt technician’s job easier, but reusing sampler tubes can allow for significant cost savings.
Choosing immersion samplers can be challenging. To learn more about immersion samplers, or to see which immersion sampler is the best fit for your application, reach out to SYSCON’s team of experts.