Technically heat input has nothing to do with cooling rate. In my opinion thats why the answer is E. Sounds like one of the AWS trip up questions. Remember the BEST answer, watch out don't read into the question to much.

Hi Tom,
I'm going to go along with scrappy in that "heat input" and "preheat" are two separate things. Preheat does decrease the cooling rate and decreases the likelihood of cracking problems.

At first glance I would tend to agree with you if the base metal being welded is a carbon or low alloy steel. The same argument will not stand if the base metal is aluminum or other nonferrous metal.

Preheat increases the temperature of a relatively large mass of base metal. Once preheated, it would take several minutes for the work piece to cool back to ambient temperature. On the other hand, increasing the heat input also increases the temperature of the surrounding base metal, but it might not necessarily heat the same mass as the preheating operation.

Compare two situations where a tack weld is deposited. The first case is a tack weld deposited with the benefit of preheat. A large volume (mass) of metal is heated by the preheating operation. It will take several minutes for the tack weld and base metal to cool to ambient temperature. Consider the same tack weld made on thick plate without the benefit of preheat.  The cooling rate of the tack weld, regardless of the heat input is going to be very high. The welder can almost immediately place his hand on the tack weld on the thick plate without fear of being burned. The potential for forming martensite increases with increased cooling rates as experienced in the latter case.

The preheat must heat a sufficient mass of base metal to ensure the cooling rate is low enough to prevent the decomposition of austenite to martensite. If the welder adheres to the recommendations of the applicable welding standard or WPS, the temperature and the mass of base metal heated is sufficient to ensure matensite will not form. The Continuous Cooling Curve (different CCC for each steel alloy) indicates what cooling rate is needed to ensure the austenite does not decompose into martensite.

As discussed, the tack weld will have a very high cooling rate if it was deposited on a thick part without the benefit of preheating prior to striking the arc. Now consider a joint in a plate that is welded with high heat input. The total energy required to deposit the weld may heat a mass of base metal sufficiently to ensure slow cooling if the plate is relatively thin, but there is no assurance that is the case. The same scenario using very thick plate will not introduce enough energy to heat up sufficiently large mass of base metal to prevent rapid cooling. In other words, the high heat input will encourage lower cooling rates, but there is no assurance that will be sufficient to prevent the formation of hard, brittle microstructures.

In contrast, a weld made with sufficient preheat, i.e., in accordance with the welding standard, is adequate to ensure the cooling rate is low enough to prevent the decomposition of the austenite (carbon steel at high temperature with all the carbon in solution) does not result in the formation of martensite.  Slow cooling is assured whether the weld is a small tack weld or a weld extending from one end of the joint to the other.

Back to your question: High heat input alone will not ensure the cooling rate will be sufficient low enough to prevent cracking. High preheat is intended to ensure slow cooling to prevent cracking. So, I would be forced to agree with response "e" for the question on heat input and response "b" for the question regarding preheat.

Al

tom cooper

Increasing heat input and increasing preheat

will decrease cooling rate everytime

But that is all part of the testing process with AWS

these increases may not effect cracking problems

The answer is E none of the above to both for unconditional answer.

Just my ¢¢'s

Marshall

Here is a good read http://www.jflf.org/pdfs/papers/keyconcepts2.pdf

Here is the first part on preheat  http://www.jflf.org/pdfs/papers/keyconcepts1.pdf

Hi Tom

This is the reason I do not like multiple choice questions. Often there are answers that may be correct in certain circumstances, but not necessarily as a general case, or sometimes there are two different answers that are correct under different circumstances etc. As a studen it is impossible to know what the examiner had in mind when asking the question.

Personally for for Q7-12 I would agree with you that the answer is (b), although this is not always the case. If the option for (b) was: "decreases the cooling rate and therefore ALWAYS decreases the likelihood of cracking problems" then I would say this is wrong, and go for (e), but most of the time this statement would not be wrong, because they are talking about "likelihoods". In other words this is ambiguous. Unfortunately there are too many such answers in multiple choice answer exams. This format is however easy for the examiners to mark, so they prefer to ask such questions rather than the "essay" type answers which take much longer to read and answer. (I know, I have set such papers myself, and then need to change my "model answers" when it becomes apparent that the question was ambiguous to the students.)

I do not want to change the subject here, but I do want to point out that the whole "heat input" thing is a generalisation that is mostly very usefull, but can sometimes lead to erroneous assumptions. (I had a quick look at the article that was attached to one of the other answers, and saw that they also dealt with the heat input issue in this "generalised" manner that can hide some deeper issues.) To explain what I am talking about, keep in mind that the same heat input can be achieved by high energy parameters and faster travel speeds as that from low energy parameters and low travel speeds. While the heat input will be the same, the high energy parameters with fast travel speeds will give higher temperature gradients, which generally results in greater penetration, narrower HAZ's and faster cooling rates than the situation with low energy parameters and slow travel speeds. - Just something to keep in the back of the mind.

Regards
Niekie

Hi again Tom,
Let me add something to help clear up my initial answer and why I think the answer "e" is correct in the first question.

AWS A3.0 Standard Terms and Definitions

Heat input = The energy applied to the work piece during welding.
Preheat = The act of applying heat to the work piece(s) prior to joining.....<snip>
[the bold emphasis is mine]

AWS D1.1:2010 Table 4.6(9)a shows the formula to figure heat input in joules per in.

Now that we've got the terms and definitions out of the way for preheat and heat input.

AWS D1.1:2010 Clause 3.5 The preheat and interpass temperature shall be sufficient to prevent cracking.<snip>
AWS D1.1:2010 Commentary C-3.5 The principle of applying heat until a certain temperature is reached and then maintaining that temperature as a minimum is used to control the cooling rate of weld metal and adjacent base metal<snip>...........The higher preheat temperatures result in slower cooling rates. When cooling is sufficiently slow, it will effectively reduce hardening and cracking.<snip>.....The amount of preheat required to slow down the cooling rate so as to produce crack-free, ductile joints will depend on:
1) ambient temperature
2)Heat from the arc...[heat input]
3)heat dissipated from the joint
4)chemistry of the steel(weldability)
5)hydrogen content of deposited weld metal
6)degree of restraint in the joint

Anyway....if you read all of the answers for the first question, none of them are exactly right as they are written....so to take each answer at face value I ended up with answer "e" because they don't fit exactly.

Second question has an answer that is correct as written, so I chose "b", in that case.

AWS test questions are written this way numerous times throughout the exam and it's easy to be tripped up by this. While heat input can in some cases be sufficient enough to prevent cracking as in single pass fillet welds on thin materials, it isn't always the case. The thicker the materials being welded the more heat is required to keep the base metal from quenching the freshly deposited weld metal and cooling it off too quickly to prevent cracking.