316L stainless steel is a kind of typical alloy that has Ni and Cr in its components. it is a low carbon steel in 316 stainless steel series and is much widely used in industry than other two alloys in our material library, so that 316L stainless steel is chosen to be gave characterization data.

Compared with 304 stainless steel, another popular type of metal material, Mo is invented in 316 stainless steel series, which improves its corrosion resistance in high temperature consistence. 316L stainless steel is also noted as 022Cr17Ni12Mo2 in China standard GB/T 20878-2007. Generally, as a low carbon steel, the carbon constant is required as ≤0.03wt%, while the 316L stainless steel usually consists 10~14wt% Ni and 2~3wt% Mo. As the corrosion resistant element, Cr is added of 16~18wt%. therefore, people defined 316L as a nichrome alloy.

316L is an Austenite stainless steel, but other phases also exist with a small area. During manufacturing, 316L usually experience solid solution strengthening and cold work strengthening, annealing is also implemented sometimes for a better chemical corrosion resistance. Those small phases precipitated on the Austenitic interface could affect the properties of 316L significantly.

Previous pages show more information about 316L stainless steel and other alloys like Inconel690 nichrome, which use more Ni and Cr than 316L. An 316LN bar is introduced as a kind of 316L stainless steel that has an extra limitation about the content of N in order to regulate its properties better for using in special constructions, and a bar shape for tests to its mechanical properties.

Introduction to Metallographic Microscope

Metallography mainly refers to the branch of materials science that analyzes, studies, and characterizes the microstructure, low-power structure, and fracture structure of materials. Both qualitative and quantitative characterization of the microstructure of materials can be obtained and that mainly reflects and characterizes the number, morphology, size, distribution, orientation and spatial arrangement of the phase, microstructure components, grains, nonmetallic inclusions and even some crystal defects.

Metallographic microscope is an optical microscope that togethers photoelectric conversion technology and computer image processing technology. By means of metallographic microscope system, not only microscopic observation on the eyepiece, also can be observed in the computer display screen, real-time dynamic images, images and will need to edit, save and print.

Principle and Accuracy of Metallographic Microscope

When using the metallographic microscope, the objective lens and the eyepiece are correctly selected and installed on the objective holder and the eyepiece cylinder respectively according to the magnification required for the sample observation. Align the center of the platform with the center of the objective lens and place the prepared sample in the center of the platform, with the observed surface of the sample facing downward. Insert the light bulb of the microscope into the low-voltage transformer and plug the transformer into the 220V power socket to make the light bulb bright.

The ideal object image can be obtained by adjusting the aperture diaphragm and the field of view diaphragm and selecting the appropriate filter. In mechanical experiments, the general metallographic microscope is mainly used to measure the metallographic structure of metal surface. By means of a metallographic microscope, one can analyze the relationship between the microstructure of steel and its chemical composition.

The structure of various steels after different processing and heat treatment can be determined. Qualitative analysis is performed on various materials to determine the steel quality, such as the distribution and quantity of various steel inclusions -- oxides, sulfides, etc., and the size of metal grains. Due to circular hole diffraction, the ultimate resolution of optical microscope is 0.2 micron. The measurement error mainly comes from the positioning error of objective lens converter, the error of objective lens replacement, load table rotation center offset and the error of eyepiece. So, optical microscope is precise but not accurate.

Example Data and Analysis ^[1]

The main component of 316L stainless steel is normal temperature austenite, however, ferrite is first precipitated during solidification, thus the residual ferrite is the main impurity in the metallographic diagram.

The metallographic images of 316 stainless steel shows how the phase distribution is in micro scale. People could know contents and morphology of ferrite in different parts were different, and the morphology of residual ferrite in the surface layer is skeletal, while the corner specimen has small grain. Since the grain size of 316L could be changed a lot during heat treatments, metallographic microscope is very basic in study to 316L. In the study by J. Tian, J. Pan, X. Chen, and J. Wang (2013) ^[1], they found that the morphology of residual ferrite in the center is striation or discontinuous skeletal shape, distributing dispersedly, and the ferrite amount of triangle area is significantly higher than the other parts of the slab up to 15.17%.

Figure 1. The micro-structures of 316L slab. ^[1]

Introduction to SEM ^[2]

Scanning electron microscopy (SEM) is a model of an electron microscope that scans a surface with light from a concentration of electrons to produce an image of a sample. Electrons interact with the atoms of the sample to generate a variety of signals, including surface topography and composition information of the sample. The second wave can detect the number of electrons, so the signal strength depends on the topography of the specimen. SEM can achieve better decomposition capability than 1nm. Views are observed at a wide range of low or high temperatures using special instruments under existing SEM high vacuum or variable pressure or ambient SEM low vacuum or humidity conditions. The error for SEM is less than 0.5%, which means it can show the surface topography and composition of the sample accurately.

Principle and Accuracy of SEM ^[2]

Figure 2. Sketch of SEM. ^[2]

In SEM, the electron beam radiates heat from the tungsten filament pole to the electron beam. The beam typically uses an energy of 0.2 to 40kev and focuses on one or two consuming lenses, about 0.4 to 5nm in diameter. The beam usually deflects the beam along the X and Y axes through the deflecting plate or the deflecting plate pair of the electron column in the final lens, scanning the grating along the rectangular area of the sample surface.

When interacting with the primary electron beam, the sample was decorated with gems known as energy-losing repetitive random scattering absorption within the electron interior, extending about 2,200 times, below 100nm on the volume dialogue volume of the specimen. The amount of interaction depends on the landing energy of the electron, the specimen and the density of the specimen. The energy exchange between the electron beam and the sample can be obtained by elastic scattering of high-energy electrons, inelastic scattering of secondary electrons, and radiation of electromagnetic waves, and can be detected by special detectors. The beam current absorbed by the specimen can also be detected to generate an image of the current distribution of the specimen. Use various types of electronic amplifiers to amplify the signal and display it in various brightness on a computer monitor. Each pixel in the computer video memory is synchronized with the position of the specimen beam in the microscope to obtain an image and a distribution of the intensity of the signal emitted from the specimen scanning area. Although images on film were taken with ancient microscopes, modern instruments mostly collect digital images.

Example Data and Analysis ^[3]

The finer microscopic phase structure can be obtained by SEM. The ultrastructural structure of 316L stainless steel is mainly composed of austenitic phase and ferrite phase. That can be observed directly. In figure 3, the black island or banded ferrite phase is distributed on the gray austenite matrix. This could tell people how material performs in the micro scale, which leads to different properties.

SEM is commonly used with Electron Back Scattered Diffraction (EBSD) in order to find position and amount of different phase, that could help people understand the SEM figure. But the components of each phase still require other techniques to obtained. ^[4]

Figure 3. SEM images of 316 L stainless steel with different aluminum content and deformation. ^[3]

Introduction to EDS ^[5]

EDS is an analytical method for element analysis and chemical characterization. The method collects the X-ray generated by the X-ray machine or other X-ray source and analyzes the X-ray emitted when the sample interacts with it. Because different elements have different emission spectra due to different atomic structures, different components in the sample can be resolved by analyzing the X-ray spectrum.

Principle and Accuracy of EDS

To excite the characteristic X-ray of a sample, a high energy electron beam, proton beam or X-ray is usually needed. First, the incident electrons (or protons, photons) excite the inner electrons of the ground state atoms. The inner electrons leave the atoms with holes. When the electrons in the outer layer and in the higher energy level fill these lower energy level holes, the excess energy may be emitted in the form of X-ray. The energy dispersive X-ray spectrometer collects and measures the energy and intensity of these X-rays. Since the energy distribution of these X-rays can reflect the atomic characteristics of specific elements, the energy dispersive X-ray spectrum can be used to determine the element composition in the sample ^[5].

The minimum content of EDS analysis is 0. X%. For the main elements (> 20wt%) with medium or above atomic number and no overlapping peak, the relative error of quantitative results is generally better than 2%, and the detection limit is 0.1% - 0.5%; the smaller the atomic number is, the lower the content is, the larger the quantitative error is, and the maximum is 50%.

Elements with 3wt% < content < 20wt%, with relative error less than 10%.

Elements with 1wt% < content < 3wt%, with relative error less than 30%.

For elements with 0.5wt% < 1wt%, the allowable relative error is within 50%.

Figure 4. Principle sketch of EDS. ^[5]

Example Data and Analysis ^[6]

Through EDS analysis, it can be concluded that Fe, Cr, Ni, Al, Mn, Mo are the main alloying elements of 316 L stainless steel. Cr and Al are the forming elements of ferrite. Ni is the element stabilizing the austenite. All of the elements have some degree of segregation. Al, Mn, Mo as the solid solution component elements, most of them solid solution in austenite and ferrite phase.

Figure 5. EDS composition analysis of aluminum-containing 316L stainless steel with different deformation quantity  (relative error 10-15%, 650℃) ^[6]