[ Wiki ]Metallography of Stainless Steels

INTRODUCTION

Stainless steels are referred to corrosion-resistant steels that consist of up to 11% chromium. This set of high alloy steels are further divided into four categories; austenitic, martensitic, ferritic, and austenitic-ferritic (duplex) stainless steels (Figure 1). These categories describe the microstructure of an alloy at room temperature, which is considerably affected by the composition of the alloy.

Figure 1. Duplex steel etched electrolytically with 150x40% aqueous sodium hydroxide solution, showing blue austenite and yellow ferrite.

Corrosion resistance is the main property of stainless steels, and this feature can be further improved by adding certain alloying elements. Such elements impart positive effect on other properties, like oxidation resistance and toughness.

Titanium and niobium, for example, boost resistance against inter-granular corrosion as they take in the carbon element to produce carbides; nitrogen to increase strength, and sulfur to increase machinability because it form tiny manganese sulfides that lead to short machining chips. Stainless steels have excellent surface finishes and corrosion resistance, so they play a major role in the medical, aircraft, food and chemical industries, in architecture, professional kitchens, and jewelry.

Metallography of stainless steels is an important part of the overall quality control of the production process. The important metallographic tests are as follows:

• Detection of delta ferrite and sigma phase
• Measurement of grain size
• Assessment and distribution of carbides

In addition to this, metallography is also utilized in failure analysis of oxidation and corrosionmechanisms.

DIFFICULTIES DURING METALLOGRAPHIC PREPARATION

Grinding and Polishing

This involves the deformation and scratching of austenitic and ferritic stainless steels (Figure 2); inclusions and carbides are retained.

Figure 2. Austenitic steel, color etched (Beraha II).

Solution

Alumina or colloidal silica can be used for systematic diamond polishing and final polishing.

PRODUCTION AND APPLICATION OF STAINLESS STEEL

High alloy steels are produced by melting and remelting processes, which are highly advanced procedures. In an electric arc furnace, a combination of well sorted scrap and iron is initially melted and continuously cast into billet or bloom, or cast into ingot form. These main products can be additionally processed into rod, bar, or plate shapes in a large number of applications. For higher quality steels, the main product can be utilized as feedstock for a secondary process of steelmaking. This process can be remelted twice or even three times by vacuum induction melting and electroslag remelting or vacuum arc remelting, which can be carried out under protective and pressure gases.

The secondary process is usually done to reduce impurities like silicates, sulphides, and oxides, so that with repeated remelts the level of cleanliness increases, producing uniform ingots with excellent physical and mechanical properties.

Application

Stainless steels’ high corrosion resistance depends on the creation of a passive surface oxide layer that spontaneously rebuilds itself when it is damaged mechanically, and is also based on alloying iron with chromium. Stress, pitting, intercrystalline, and vibrational corrosion are different types of corrosion that can occur. If alloying elements other than chromium are added, better resistance against certain forms of attack can be obtained. Molybdenum, for example, enhances resistance against pitting corrosion. Here the primary alloys, properties, and the associated applications of four forms of stainless steels are elucidated.

Ferritic stainless steels have a low carbon content, with 11 to 17% of chromium, and they are non heat treatable alloys. Properties of ferritic stainless steels include moderate strength and toughness, magnetic property, and resistance to atmospheric corrosion. Applications include car trim, razor blades, and magnetic valves.

Martensitic stainless steels have a medium carbon content, with up to 12 to 18% of chromium and 2 to 4% of nickel. They are heat treatable alloys. Properties include high creep resistance, high temperature resistance, and high corrosion resistance. Applications include knives, scalpels, tweezers and hooks in medical applications, high performance parts and drive systems for aircraft.

Austenitic stainless steels have 0.03 to 0.05% of carbon and their main alloying elements are molybdenum (2-4%); nickel (8-25%), and chromium (17-24 %). Niobium and titanium are added for carbide forming. Austenitic stainless steels are not heat treatable. Properties include high corrosion resistance, high ductility, good cold forming properties, resistant to oxidizing acids and alkalis, and easy to work and machine. Applications include implants, bolts, and screws; low temperature applications comprise pipes and vessels and pipes in the food, chemical, and pharmaceutical industries, and kitchen utensils.

Austenitic-ferritic steels (Duplex) have lower nickel content (4-6%) and higher chromium (21-24%). They have 2 to 3% of molybdenum and exhibit a low carbon content. Properties include excellent resistance from stress corrosion, and fatigue resistance in corrosive media. Applications include architecture, equipment for environmental, chemical, and offshore industries.

Difficulties in the Preparation of Stainless Steels

Austenitic steels are ductile and ferritic stainless steels are soft, and both are inclined to mechanical deformation. These steels become highly reflective when subjected to final polishing, but if they are not fully pre-polished, deformation will reappear following etching (Figure 3). Martensitic steels have excellent hardness, and can be easily polished. However, carbides should be preserved properly.

Figure 3. Austenitic steel insufficiently polished 500x showing deformation after etching (Beraha II).

RECOMMENDATIONS FOR THE PREPARATION OF STAINLESS STEELS

High pressures and highly coarse grinding papers or foil should not be used for soft and ductile stainless steels, as this can lead to deep deformation. Generally, the finest possible grit, which is uniform with the surface roughness and sample area, must be utilized for plane grinding. Diamond on a rigid disc, such as MD-Largo, is used to perform fine grinding, or on a MD-Plan cloth as an alternative to certain types of stainless steels. After fine grinding, a complete diamond polish is done on a medium soft cloth. This is followed by a final polish using alumina (OP-A) or colloidal silica (OP-S) for scratch removal. This particular step should be done meticulously and will take several minutes. A better contrast can be obtained through a good final polish. Fine grinding and even final polishing will not remove deformations occurring from the initial grinding step. Such deformations will leave some traces.

A preparation method for stainless steel samples is shown in Table 1, and a preparation method for 6 stainless steel samples is illustrated in Table 2.

Table 1. Preparation method for stainless steel samples, 30 mm diameter mounted, on the semi-automatic Tegramin, 300 mm diameter

Table 2. Preparation method for stainless steel samples, 65x30 mm, cold mounted or unmounted using Struers MAPS or AbraPlan/AbraPol, 350 mm diameter

ELECTROLYTIC POLISHING

When it comes to rapid general structure check and research analysis, electrolytical polishingand etching provides an alternative option to mechanical polishing of stainless steels, because this process does not leave mechanical deformations. However, while electrolytical polishing does provide good result for investigating the microstructure (Figure 4), it is not suitable for detecting carbides, which appear either enlarged or washed out.

Figure 4. Stainless steel weld, polished and etched electrolytically, DIC

The samples need to be ground to 1000# on silicon carbide paper or foil prior to electrolytical polishing. If the initial surface is finer, better electrolytical polish can be obtained. Preparation method is as follows:

• Electrolyte: A3
• Flowrate: 13
• Area: 1 cm²
• Voltage: 35 V
• Time: 25 seconds

External etching with stainless steel etching dish

• Voltage: 15 V
• 10% aqueous oxalic acid
• Time: 60 seconds

ETCHING

Some amount of expertise and patience is required to etch stainless steels. Extensive literature is available for etchants, and it is suggested to test out various etchants to set up a separate stock of solutions that are suitable for a certain material prepared in the lab on a regular basis.

Stainless steels have excellent resistance against corrosion, so very strong acids are needed to expose their structure. When handling these etchants, standard safety precautions should be followed. In most labs, the etchants specified in the literature will be altered based on personal preference or the material that is being etched. Adequate final oxide polishing is required to obtain good etching results. Some etchants are effective in routine applications, and they are as follows:

Chemical etching

• For martensitic steels - 25g picric acid, 925 ml ethanol, 50 ml hydrochloric acid.
• For austenitic steels - Swab etch: 500 ml distilled water, 300 ml hydrochloric acid, 200 ml nitric acid, 50 ml of a saturated iron-III-chloride solution, 2.5g copper-II-chloride, 300 ml hydrochloric acid, 100 ml water, 15 ml hydrogen peroxide (30%); V2A etchant: 100 ml hydrochloric acid, 100 ml water, 10 ml nitric acid, etch at room temperature or up to 50°C temperature.
• Color etchant Beraha II: Stock solution, 800 ml distilled water, 400 ml hydrochloric acid, 48g ammonium biflouride; to 100 ml of this stock solution 1 to 2g of potassium metabisulfite should be added for etching.

Electrolytic etching

• All stainless steels: 10% aqueous oxalic acid
• For austenitic-ferritic steels (Duplex) - 40% aqueous sodium hydroxide solution

The proposed safety precautions should be followed when handling chemical reagents.

STRUCTURE INTERPRETATION

Heat treatment has no effect on ferritic stainless steels, but the properties of these steels can be affected by cold working. At room temperature, ferritic stainless steels are magnetic. In annealed condition, the microstructure includes ferrite grains, wherefine carbides are integrated. Ferritic steels employed for machining purposes include a considerable amount of manganese sulfides to enable free cutting, as shown in Figure 5.

Figure 5. Ferritic stainless steel with manganese 200x sulfides and strings of small carbides, etched electrolytically with 10% oxalic acid.

Heat treatment has a major effect on martensitic stainless steels, which are formed via instant cooling. Tempering treatment can be used to optimize their properties. The alloys of martensitic stainless steels are magnetic in nature. The microstructure can range from pure martensitic structure, through to fine tempered martensite based on the thermal treatment. Complicated heat treatment temperatures are required for different alloys and different sizes of semi-finished products. An often unwanted phase is delta ferrite (Figure 6), because extended annealing times of steels with high chromium content at 700 to 950°C can alter the delta ferrite into a brittle and hard iron-chromium intermetallic sigma phase.

Figure 6. Tempered martensitic stainless 75xsteel with delta ferrite, etched with picric acid.

The sigma phase and the embrittlement are removed by heating to a temperature of 1050°C. Thermal treatment has no effect on austenitic stainless steels, but quick cooling leads to the formation of their softest condition. Austenitic stainless steels are non-magnetic in this state, and their properties are affected by cold working. The steels’ microstructure includes austenite grains that may display twinning (Figure 7).

Figure 7. Cold worked austenitic steel showing twinning, etched with V2A etchant.

When these steels are exposed to increased temperatures of 600 to 700°C, complex carbides are formed inside the austenite grains. This results in an insolvency of chromium in the austenite solid solution, increasing the sensitivity to inter-granular oxidation or corrosion. The risk of inter-granular corrosion can be minimized by reducing the carbon content to less than 0.015% and introducing minute quantities of niobium or titanium. This is because these elements form carbides rather than the chrome (Figure 8). Delta ferrite can be a result of the cold working of austenitic steels or heat treatment conditions in martensitic steels (Figure 9).

Figure 8. Austenite with carbides and some 200x titanium carbon nitrides.

Figure 9. Austenitic steel with strings of delta 125xferrite, showing microsegregations. Blue areas: depletion of alloying elements.

Austenite and ferrite are present in austenitic-ferritic stainless steels (Duplex). The structure is revealed through electrolytic etching in a 40% caustic soda solution, and this helps to estimate the right percentage of individual phases (Figure 10). These steels are ductile and are mainly utilized in the paper, food, and petroleum sectors.

Figure 10. Forged duplex steel showing blue ferrite, white austenite and fine needles of sigma phase, etched electrolytically with 40% aqueous sodium hydroxide.

CONCLUSION

Corrosion resistant steels are referred to as stainless steels, which contain high contents of nickel and chromium. Stainless and ferritic steels are soft, ductile, and inclined to scratching and mechanical deformation during the course of metallographic preparation. Moreover, carbides cannot be retained often. To ensure an effective mechanical polish, the following things should be considered.

• Fine grinding and polishing with diamond must be meticulous, and all deformation should be removed from plane grinding.
• Coarse abrasives for plane grinding should not be used
• A final oxide polish with alumina or colloidal silica should be done to provide a surface that is free from deformation.

A four step process carried out on an automatic preparation system provides good and reproducible results. Chemical etching of stainless steels can be difficult, and the proposed etchants are corrosive and have to be handled carefully. Another option is to use electrolytical polishing and etching, which does not retain carbides, but provides a deformation-free surface.