You may have heard the saying that engineering is impossible without material. Any damage to the structure of the material, which is so important for engineering, causes many problems from health problems to financial losses.
Considering that any product is created by combining many different materials, a material damage ; It indicates the incompleteness of a product or a huge waste of time. Due to such reasons, one of the issues that engineers are most afraid of is damage to the material.
Of course this damage is uncontrolledIf it happens in some way, it poses a problem. Certain procedures are applied in order to prevent material damages that occur out of control. By ensuring the working conditions to use the material to be used in the product, the material is damaged in a controlled manner . Changes and improvements are made depending on the reactions of the material. The purpose of existence of the creep experiment is based on this. Now let’s examine the creep experiment in all its details.
What is a Creep Test?
Creep is the name given to the deformation that occurs in the material due to some external factors . So what is the creep test? Creep Test and load . In creep tests, the creep limit is determined. So how is this creep limit determined? The creep limit is determined by the elongation of the material for certain time periods just before breaking at a certain temperature. If the material undergoes 1% plastic deformation during the test, the test is terminated. The creep experiment consists of three stages, let’s start to make sense with the first stage …
Primary Creep:
- Primary creep is the first stage of creep .
- The sample stretches under the effect of the load, where dislocation movements are quite high.
- The sample becomes deformation hardening.
- On the other hand, since it is at high temperature, internal stresses are removed by annealing. Recovery and recovery occurs.
- As the deformation hardening becomes more dominant, the creep rate decreases gradually, that is, the resistance of the material increases.
What is this dislocation? Dislocation, also known as linear flaw ; are defects seen along a line in the atomic arrangement of crystal structures in materials science.
So, what is the reason for the excessive dislocation movements at this stage ? At the beginning of the experiment, when the first load is applied, the shape change occurs until the material has a stable structure in shape . Due to this, dislocation movements increase. Increased dislocation movements mean dislocation accumulation and material strength . Apart from this, decreases occur in stresses in primary creep. The biggest reason for this is temperature. This phase is not taken into account in calculations. The reason for this is the unstable plastic shape changes that occur .
Secondary Creep
- Another name for this zone is the stable creep zone.
- In this process, the slope and creep rate of the curve are constant.
- The stress that causes stable creep is called creep resistance.
- This is the longest stage of the creep process .
- This is because there is a balance between deformation hardening and recovery .
- With the recovery, the material softens partially and may undergo further deformation.
- It is also called viscous creep and hot creep as it is encountered only at higher temperatures .
- Creep regime is applied because there is excessive narrowing in the cross section area.
The reason for the constant creep rate is that the hardening mechanism activated by dislocations and the recovery mechanism are balanced with each other in this region. Perhaps the factor that makes secondary creep so important is the slope of the linear line consisting of this region, giving the material creep speed.
Tertiary Creep
- It is the third and final stage of creep .
- At this stage, an increase in creep rate is observed.
- In this process , events such as internal cracks, voids, grain boundary separations, necking occur in the material.
- There is a serious decrease in cross-sectional area due to necking and the formation of internal spaces.
- At the end of these developments, the rate of deformation increases rapidly and rupture occurs at the end of this process .
How to Perform a Creep Test?
First, samples of the material whose creep properties are to be determined are produced in accordance with the upper and lower jaws . The samples obtained are placed in the upper and lower jaws by opening the oven door. After it is placed, the oven door is closed and when the desired temperature is reached, a load is applied to the sample and time is kept with the help of a stopwatch. So what is the desired temperature value? The desired temperature is 450 ° C . Recording is done by an electronic recording system . This recording system can detect the elongation and temperature changes in the sample at desired time intervals thanks to a 100 mm wide print paper reports the .. The report issued by the registration system is checked periodically. According to the report, if there is a deviation in the temperature values, the test is canceled immediately. The reason for this is that the creep test is an experiment that has to be done at a constant temperature . If a deviation is found in the elongation data when the report is examined, it means that the sample is either broken or has exceeded its working dimensions. In addition, the elongation-time graph is drawn using the data obtained from these reports . Since the test period will be very long, uninterruptible power supply should be operated against instantaneous power failure that may occur, albeit rarely .
Factors Affecting Creep
There are four main topics that affect creep. Let’s examine them in more detail …
Grain Size Effect
- What is the most important factor affecting creep? If a question comes up, we can answer the grain size .
- Materials with smaller grain sizes are more durable than materials with larger grain sizes .
- The world of science is full of exceptions, one of those exceptions is here as well. If the conditions are above the eco-adhesive temperature, the opposite happens. In other words, materials with larger grain size become more advantageous.
Alloying Effect
- With the addition of the alloying element, the formation of hard carbides in the material is achieved.
- Thus, dislocation movements are prevented.
- The creep resistance of steel increases with elements such as Ni, Co and Mn remaining in solution and carbide forming elements such as Cr, Mo, W and V.
Melting Temperature Effect
- First of all, it is useful to say that the melting temperature and activation energy follow a directly proportional graph.
- The grain boundaries of materials with high melting temperatures are more durable at high temperatures.
- As a result, materials with high melting temperatures have higher creep resistance.
Microstructure Effect
- In the material designs that we set out to be resistant to creep , directional grain microstructure method produced by directional solidification method should be used.
- The microstructure shape, in which we see the highest creep resistance , is single crystal directional compared materials.
What Are Creep Mechanisms in Metals?
The creep mechanisms of metals are collected under three main headings.
- Dislocation slipping and climbing
- Diffusion Flow
- Grain Boundary Shift
Now let’s examine it in more detail …
Dislocation Slip and Climb
It is necessary to reach high temperatures in order to talk about dislocation climbing . So what is dislocation climbing? As an answer to the question, dislocations reaching high temperatures leaving the plane as a result of spreading to empty places in the grain is called dislocation climbing. Another striking feature of dislocations is that they continue to slide while climbing away from irregular areas. In the scientific world, as it is a result of every action, it becomes inevitable that the material shifts away from the irregular area in its dislocations , and the deformation of the material even in the smallest stress applied by the external force .
Diffusion Flow
Considering the necessity of the conditions, we see that it has a similar structure with the dislocation climbing . Diffusion flow is a creep mechanism that occurs under high temperature conditions , just like dislocation climbing . As the temperature increases, the force that holds the grains together decreases, so the stress on the material and the grains are separated from each other by diffusion .
Grain Boundary Shift
While the grain boundaries are immobile and prevent dislocations at low temperatures, the grain boundary slip mechanism becomes active in high temperature conditions. As the temperature increases and the deformation decreases, the forces holding the grains together decrease and the grains slide over each other with the effect of stress. The role of this mechanism increases as there are many grains and abundant grain boundaries in fine-grained materials. Therefore , it would be more advantageous to use large-grained materials at high temperatures . The reason why Ni (nickel) based super alloys monocrystalline are used in the turbine blades of jet engines is based on this.
Result
Creep is the time-dependent deformation of metals under a constant stress at temperatures above 0.4 Tm . A typical creep curve consists of three regions . The most important region where important information is obtained from creep curves for engineering applications is known as the second region. Another name creep stable , known as the slope in the second region; creep rate and breakage values are obtained. Selection of alloys to be used at high temperatures is a very important issue. These alloys include Ni and Co based superalloys, stainless steels and refractory we can count metals. The primary purpose of the creep test is to provide the working conditions in which the material will be used, and the material is damaged in a controlled manner . Changes and improvements are made depending on the reactions of the material. Choosing the right material is vital in engineering applications . Thanks to the creep test , the right material selection is made much easier.
