Eric Fitterling November 30, 2010

Sensitization in Stainless Steel

Abstract

Stainless steels, especially austenitic types, are susceptible to a phenomenon known as sensitization at

elevated temperatures between 500-850° C.1 From this, intergranular corrosion can occur and cause

failures. Compositional adjustments such as additions of nitrogen, titanium, or niobium can alter the

driving forces behind sensitization. Since this process only occurs in specific temperature ranges, it

occurs during specific processes such as welding, but can be avoided using careful planning.

Deformation can lead to different properties including desensitization. Sensitization is a unique

occurrence and there is still not a perfect test to detect and classify the degree of it. The definition,

prevention, and intricacies of this event are discussed.

Introduction

Sensitization is a major problem in stainless steels that affects the alloy’s durability. Chromium additions

in steel are what make the alloy “stainless” and it is the main contributor to sensitization. Stainless steel

(SS) is defined by the relative amount of chromium in its composition. It must include more than 10.5

weight percent (wt %) chromium.1 Other elements are added to yield different properties, such as

nickel, which can stabilize the austenitic (face centered cubic) structure of steel at room temperature.

Ultimately, it is the chromium content that makes the steel stainless. Chromium is extremely reactive

with oxygen and will form a very thin chromium oxide layer on the surface of stainless steel. The film

that is created is on the order of nanometers in size and is what protects the underlying metal alloy from

corrosion and further oxidation.1

Chromium allows stainless steels to be resistant to harsh environments, but as stated, it is also why

sensitization occurs. At elevated temperatures chromium carbides

precipitate at grain boundaries (Figure 1). These carbides (Cr23C6) are

much harder than the rest of the alloy which can weaken the overall

structure. Additionally, the chromium carbides form by taking

chromium atoms that are already at the grain boundaries. This

leaves these regions with low chromium content. The localized

depletion of chromium means the chromium oxide film that protects the alloy can no longer form.2

1

Figure 1. A micrograph of 316L SS after a heat treatment at 700° C for 16 hours. Carbide formations are apparent in black at grain boundaries.1

Sensitization is a phenomenon that occurs in all four types of stainless steels, but it is important to note

that it affects each type differently. The first and most widely used stainless steel is austenitic SS. This

variety of SS has a face centered cubic crystal structure, known as austenite, which is made possible

through nickel additions. Ferritic SS, on the other hand, is body centered cubic in crystal structure and

has little to no nickel in it. Martensitic SS is similar to ferritic SS in composition, but it has high levels of

carbon sometimes above 1 wt %. Lastly, there is duplex SS which incorporates both the ferritic and

austenitic structures together.3

Alloy Composition

As previously stated, steel that has over 10.5 wt % chromium is deemed a stainless steel. In reality most

grades of stainless steel include 16 to 25 wt % chromium. Sensitization occurs when the chromium

depleted zones (near grain boundaries) fall below 12 wt % chromium. At this point, the alloy becomes

very susceptible to intergranular corrosion, which entails preferential attack of grain boundaries and

surrounding areas.2 Sensitization is directly related to the depletion of chromium to form carbides. The

direct solution to the problem would be to lower the content of carbon. Since chromium is the solution

to steel’s corrosion problems, carbon is the only element that can be limited in quantity. Carbon levels

generally lower than .030 wt % leave alloys with an insufficient amount of carbon to form chromium

carbides. 4 Unfortunately, this low amount of carbon can lead to considerably weaker properties in the

steel so other elements need to mitigate the impact carbon content plays on strength.

A study performed on 316L SS by Parvathavarthini and Dayal examined the effects of limiting the carbon

content in favor of nitrogen additions. Nitrogen assumes carbon’s role in the steel lattice to allow it to

keeps its strength. Their study included SS plates with nitrogen contents ranging from .07-.22 wt %

nitrogen and .025-.035 wt % carbon. The experiment showed that sensitization in nitrogen containing

316L SS occurred in the temperature range of 550-700° C. This is a much smaller range than the

generally accepted sensitization temperature range of 500-850° C found in austenitic SS. Nitrogen

content did not alter the temperature range, but it did affect the critical cooling rates of each sample.

Parvathavarthini notes that their experiment agrees with other literature stating that, “above 0.16 wt %,

nitrogen addition is detrimental from sensitization point of view.” At this point, chromium nitrides

(Cr2N) can precipitate and result in sensitization. Cooling rates are extremely important when

considering the cooling of stainless steel after heat treatments and after welding.1

2

Instead of reducing carbon content, other elements can be added to stabilize higher carbon content and

allow steel to maintain its strength. In U.S. patent 4,408,709 Thomas M. Devine, Jr. describes how

certain alloying additions retard sensitization in stainless steels. Elements such as titanium or niobium

reduce sensitization effects in SS because they form carbides preferentially over chromium. The

addition of these elements will not guarantee a SS is resistant to sensitization. Titanium additions raise

the brittle-ductile transition temperature. In excess, titanium can lead to embrittlement in the alloy and

incorrect heat treating, even with the proper additions, could still cause sensitization. Devine’s claim, in

his patent, is the amount of titanium that is a happy medium between embrittlement and sensitization

resistance for ferritic stainless steels used

in pre-heater and re-heater components

of steam-driven power plants. It also

included the compositions of all the

elements, as shown in Figure 2. Titanium

can be used to stabilize other types of

stainless steels, but the specific amount

required to prevent sensitization and

keep from embrittlement is very alloy

specific.4

When Sensitization Occurs

Sensitization occurs, as discussed, during a temperature range well above room temperature. Different

types of stainless steels have different ranges and different alloys will as well. In any case the

sensitization temperature region is encountered primarily during typical operation, heat treating,

welding, and start-up/shut-down. The onset of sensitization depends not only on temperature, but also

on time spent in that range.

Typical operation and heat treatment depend on soak temperature and time. This is why metallurgists

have created time-temperature-sensitization diagrams. Devine’s patent for re-heater and pre-heater

piping in steam-driven power plants is a prime example of the typical operation case. Sensitization

occurs in a specific temperature range, and if a part is to operate within that range, then it has to be

protected. As is the case in any diffusion-based phenomenon, sensitization depends directly on time. It

takes time for atoms to diffuse out of solution and to form together into precipitates. In the case of

3

Figure 2. Recommended composition for a ferritic SS to be used in a re-heater/pre-heater application.4

typical operation, the time to sensitization for an alloy needs to be years. When referring to a heat

treatment, the time to sensitization needs to be longer than the soak time. Obviously, if typical

operation temperature range can avoid the sensitization temperature range there is no problem. It is

also relevant to point out that anneals will never result in sensitization because the anneal temperature

is high enough where all of the elements are in solution. Figure 3 shows an experimental time-

temperature-sensitization

diagram of type 316L SS with .22

wt % nitrogen and .035 wt %

carbon.1 If this material were

used in an application, it would

need to operate well below the

550° C threshold displayed in

this chart at 100 hours. This

temperature can, however, be

used for heat treating, as long as

the soak time is much less than

100 hours.

Cooling rates are relevant in heat treatments as well. If an anneal is carried out at a high temperature

and allowed to cool slowly enough that it reaches the sensitization region, then the anneal was

worthless. During typical operation, a part may operate above the critical sensitization region, but when

shut down occurs, it may cool slow enough to become sensitized. Alternately, when the part is being

started again, it may rise slowly to the operational temperature and also be exposed to the sensitization

range. The process, however, that is most susceptible to sensitization is welding. If a weld is performed

at too high of a temperature or does not cool fast enough, then the metal could sensitize. Generally,

welding of stainless steels is carried out at low temperatures to avoid sensitization. It is recommended

that “…when welding stainless steel to use low heat input and restrict the maximum interpass

temperature to around 175° C…” Also, relieving stresses should be performed at or below 450° C.5 Most

sensitization problems, including welding, can be avoided by using low temperatures. When an area is

to be extensively worked on, the area can be allowed to cool when its temperature rises too high.

4

Figure 3. Experimental time-temperature-sensitization diagram for type 316L SS with.22 wt % nitrogen and .035 wt % carbon.1

Desensitization

When sensitization occurs, it can only occur to the equilibrium point between chromium carbides and

dissolved chromium and carbon. Observations have been made of deformation’s affect on sensitization

by Ramirez, Almanza, and Murr. Their study was conducted on type 316 SS with 50% uniaxial

deformation. Age cycles were carried out at temperatures of 625° C and 670° C for 100 hours and both

showed that the degree of sensitization was increased in the deformed samples as compared to non-

deformed control samples. “These differences in kinetics are due to changes in chromium diffusivity

caused by enhanced dislocation pipe diffusion of chromium…” or essentially, the dislocation

arrangement introduced by the deformation process enhanced the diffusion of chromium. 2

Deformation causes dislocations that were once isolated to interact and connect, thus allowing channels

for diffusion to take place. In the case of the sample aged at 670° C, the degree of sensitization reached

a maximum after 10 hours and then began to decrease

(Figure 4). Desensitization occurred until the end of the

test at 100 hours, at which point the degree of

sensitization equaled that of the control. This data

shows that aging will result in the same degree of

sensitization regardless of deformation processes when

enough time is allowed. The test performed at 625° C

did not show desensitization, but Ramirez points out

that the same type of convergence, in degree of

sensitization, would have occurred after the 100 hours

of aging the test was run for.2

Sensitization Analysis

Nondestructive analysis of all products being made is a very crucial part of steel production. Properties

can be checked by machining test pieces and performing destructives tests, but defects such as cracks,

seams, and tears need to be checked for throughout the entire bar. Since the bar cannot be cut up and

examined at every section, nondestructive tests must be run. The aforementioned defects can be

located using ultrasonic inspection and eddy current tests. Both involve sending energy, either sound

waves or electrical current, through the metal to observe any kind of alteration or abnormality in the

reading. Sensitization is not a problem to be taken lightly and must be tested for throughout the bar.

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Figure 4. Comparison of degree of sensitization for non-deformed and deformed 316 SS aged at 670° C illustrating desensitization.2

An experiment conducted by Stella, Cerezo, and Rodriguez used ultrasonic inspection testing on type

304 SS to observe if it could accurately detect sensitization and the magnitude of it. Specifically the

ultrasonic emissions were being measured for attenuations and their velocity. The six different heats

included a control (M1), a carbide solution treated and furnace cooled specimen (M2), a carbide

solution treated and normalized sample(M3), and three samples sensitized at 800° C for different

lengths of time (2, 6, 10 hours – M4, M5, M6). The attenuation measurements (Figure 5) obtained from

the ultrasonic testing appear to give a good sign of sensitization and its degree. The authors say that the

degree of sensitization “…can be estimated by combined measurements of attenuation and amplitude of

the main peaks on the power spectra…” but they also state that “…chromium carbide precipitation

cannot be directly identified by ultrasonic attenuation measurement.” Velocity measurements cannot

allude to the degree of sensitization.6

The experiment concerning uniaxial deformation by Ramirez et al. used a technique called

electrochemical potentiokinetic reactivation (EPR) to evaluate the degree of sensitization present in

specimens. EPR uses potentiokinetic scans that lead to characteristic peaks in the reactivation region of

polarization curves signifying sensitization. Historically this test has evaluated sensitization in standard

austenitic stainless steels, but Čihal and Štefec collected information on how EPR works on other types

of SS. The majority of tests evaluated were carried out to identify the susceptibility of different stainless

steels to intergranular corrosion caused by sensitization. EPR readings of ultra-high austenitic SS

specimens could detect M23C6 carbides and “estimate the extent of precipitation.” The martensitic SS

alloy (15Cr17Ni2) tested showed the reactivation to passivation charge ratio of the EPR test to be

different than normal. This change is vindictive of intergranular attack at austenite grains from the

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Figure 5. Variation in attenuation at 10 MHz frequency showing an increase in sensitized samples.6

tempering. A martensitic/austenitic duplex SS (Cr13Ni6Mo) also showed preferential attack at austenitic

grains. In ferritic stainless steels, EPR was used to evaluate sensitization and find the correct ratio of

carbon to nitrogen.7 Overall, EPR is a very effective test, but it is more of a laboratory test than a field

test, and cannot easily be applied to a large product such as bar or billet.

Conclusion

Sensitization is a detrimental occurrence in stainless steels that can lead to intergranular corrosion due

to localized chromium depletion and carbide formation. Alloying elements such as titanium and

niobium can stabilize SS and prevent a high degree of sensitization. Using appropriate temperatures and

cooling rates can protect against sensitization during heat treatments, typical operation, welding, and

start-up/shut-down. Steels with deformation will show a higher degree of sensitization initially at

elevated temperatures, but will eventually desensitize to a level consistent with non-deformed material.

EPR is the most reliable technique for evaluating sensitization, but lacks large scale applicability.

Ultrasonic testing has potential to estimate sensitization in stainless steels.

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Works Cited

1 Parvathavarthini, N., and R. K. Dayal. "Time–temperature-sensitization Diagrams and Critical Cooling

Rates of Different Nitrogen Containing Austenitic Stainless Steels." Journal of Nuclear Materials

399 (2010): 62-67. Electronic.

2 Ramirez, L. M., E. Almanza, and L. E. Murr. "Effect of Uniaxial Deformation to 50% on the Sensitization

Process in 316 Stainless Steel." Materials Characterization 53 (2004): 79-82. Electronic.

3 "Stainless Steels." Metallurgical Consultants. AMC, 11 Sept. 2007. Web. 27 Nov. 2010.

<http://www.materialsengineer.com/E-Stainless-Steel.htm>.

4 Devine, Jr., Thomas M. Method of Making Titanium-Stabilized Ferritic Stainless Steel for Preheater and

Reheater Equipment Applications. General Electric Company, assignee. Patent 4,408,709. 11

Oct. 1983. Electronic.

5 Dyson, John. "Austenitic Stainless Steel." GoWelding. 1 Feb. 2004. Web. 27 Nov. 2010.

<http://www.gowelding.com/met/austenitic.html>.

6 Stella, J., J. Cerezo, and E. Rodriguez. "Characterization of the Sensitization Degree in the AISI 304

Stainless Steel Using Spectral Analysis and Conventional Ultrasonic Techniques." NDT & E

International 42 (2009): 267-74. Electronic.

7 Čihal, Vladimir, and Rudolf Štefec. "On the Development of the Electrochemical Potentiokinetic

Method." Electrochimica Acta 46 (2001): 3867-877. Electronic.

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